Hmmm...okay, I can weigh in with my experience. It's long, but hey you asked. As always, for the ‘well I never had anybody honk at me when I was riding’ responders, YMMV of course.
For comparison with the iBike Pro, I have a Powertap SL and have ridden a club training tire with a Powertap Pro. I got the iBike Pro as a cheaper alternative to use on my commuter bike, which this time of year especially sees a lot of my ride time. I have always been curious to know how much training stress and recovery impact there is from the two-a-day-all-out-death-avoidance extended sprints that best describe my country-to-urban commute.
I am on my second iBike Pro unit at this point. The first one I believe had something wrong with it out of the box as it ate batteries like The Matrix. You think recalibrating one of these suckers every couple of rides is a pain, try having your unit/CPU freeze up and cut out every couple of rides from a low battery. I returned that unit and got another which I have been riding for a while now. I have gotten somewhat better results, but I definitely suspect these units to be both cold sensitive and comparative battery eaters. After several months of riding with the iBike Pro that is certainly true of my experience when compared to my experience with Powertaps. I will replace my Powertap CPU battery twice, maybe three times a year, depending upon ride and download time, and the hub batteries once a year. With the iBike Pro I am changing the battery at least once every month. In the colder months it has definitely been even like once every two weeks. And on the basis of number or rides, or duration, I can't explain or justify this. As best as I can tell there appears to be a fairly narrow window within which a new battery will put out enough juice to allow the iBike to properly function. Below that, you get either bad ride data or an error message and none at all. The weird part is that I can then take the same battery and pop it into my Polar computer or chest strap and get a lot of use from it down the road. I have done everything that I can think of to improve battery life, right down to removing the CPU from its bike mount between rides, keeping it in doors and out of the cold, etc., with no noticeable improvement. The worst part about this aspect of the iBike experience for me is that as battery life goes, so goes the units ability to hold a calibration and to produce useable, authentic ride data. Many, many, many times I have found myself at the side of the road switching out to a new battery in hopes of getting any useable data from my ride after discovering that I am no longer getting anything but gibberish displaying on my CPU.
As for accuracy and consistency in terms of data, one thing that riding with a Powertap does for you is to give you a really good idea most of the time of how much power you are cranking out, especially in terms of max and average relative to what you are doing and how you are riding. I have ridden my commute with my road bike and Powertap many times to get some general comparison data to use as a guide. For the most part, albeit with this second iBike Pro unit, I am capable of getting decent data read-outs. Part of the calibration aspect of the iBike unit though is to input the exact combined weight of you, your bike and your riding gear, right down to what you will be wearing and carrying on that ride. If you screw that part up, or don't recalibrate your unit when this combined weight and profile will vary ride to ride, you can definitely screw up the accuracy or authenticity of your read outs. So you can skew your data if for some reason you want to, or you can well be getting skewed data and not know it if you skipped the scales and just winged it at guessing the weight. Even calibrations on the hoods versus rides in the drops will give you skewed data. They make all of this very clear in the iBike literature, but it is still an aspect of the unit that bears consideration.
Overall in terms of data authenticity, I have noticed more weird ‘where the heck did that come from data anomalies’ from the iBike than I have ever gotten from a Powertap. You know how with a heart rate monitor electrical interference can give you mystery 225bpm readings? Well the iBike does the same thing a lot in my experience under max efforts. I have seen everything from speed, incline and watts being affected. The unit calibrates and calculates based upon the parameters which include combined weight and are relative to a consistent riding position during the calibration. Stuff like hopping out of the saddle and bursting into a full on sprint seem to confuse the crap out of its little brain. Doing that swinging out of a paceline (or out of the draft of a car as I have frequently seen) can be even more problematic. That is the biggest pain from these units in terms of obtaining reliable training data. When I know at best, I am dialing up to maybe 900 watts at a full on, fully loaded sprint out from a stop light on my commuter...and this is after plenty of comparative efforts with a Powertap...I get home to download my ride to find that I am the new Tom Boonen. Now granted I know that trying to sprint through an intersection before semi hits you is inspirational, but I don't believe that 1300 much less 1700 watt bursts to be anywhere in my repertoire. As a result, for those of you using CyclingPeaks, I’ll bet that I have ended up scrapping a third of my rides because they are skewed by these spikes and mystery data outliers.
Recent firmware updates have definitely taken care of a number of data cut-out, and algorithm funkiness issues that before were known to create this type of anomaly and otherwise mess with the accuracy and consistency of your ride data. Even with the latest updates, I believe these units to definitely still be sensitive to rough road issues. There are miles and miles and miles of lovely Texas chip and seal up in my neck of the woods that will kill your iBike unit data collection and read outs all together for entire extended sections of your ride. On my commute, I notice that data accuracy cuts out over even slight pavement variations, like riding over sections of asphalt on concrete pavement patches or over the bumpy transitions from sections of paved concrete multi-use paths. Stuff like pot holes or rough asphalt that I see along the city portion of my commute wreak havoc on the iBike. Even on perfect pavement, the readings cut out when cornering too. By comparison, none of this will happen with a Powertap. The newer firmware updates to the iBike have been designed in part to compensate for its issues with less than pristine pavement. But these appear to my little brain to be more in the form of how the power algorithm compensates, interprets and extrapolates for the gaps in your ride data than of truly eliminating the unit's sensitivity to bumps and variations over the road. This is a unit that requires a rock sold mount to your bike, right down the foam adhesive strips that are included in the install elements that come with the unit.
The other big let down with the iBike for me has been in terms of ease of use, and usability over the road. Maybe it’s just me, but I find the Powertap CPU to be psychotically easier to access and manipulate. The five point rocker switch approach utilized by the iBike doesn't really work over the road, especially when compared to the one finger or two finger push button approach of the Powertap CPU. And even then, with the Powertap CPU you are talking about only two raised easy-to-locate and manipulate buttons. Recent firmware updates have done some things to improve this a bit in the iBike, but that has been in terms of improved screen prompts and manipulation. The weakness in the unit for me still remains the use of a five point rocker switch. This can be a lot harder to do over the road than you would think by just looking at the unit. It is really easy to hit a bump as you press and find yourself in another ‘how the heck did I get here’ area of the standard read out menu; and the press and hold features can take multiple attempts to complete unless you are doing them over a nice smooth stretch of road with it taking several seconds to find the right portion of the rocker to press and hold.
Another big gripe of mine is that there is not a standard screen readout on the iBike which will display watts, speed and distance or trip time on the same three line CPU read out. You get two basic read out formats: Bike (speed, distance, trip time) and Power (speed, power, calories). You can get to some additional ride data as you can with the Powertap, but there is no way to toggle one of the read-out lines into showing you what you want to see in addition to the speed and power data. (For reference, the Powertap CPU reads top line watts, middle line speed and lower line you can toggle between the other data points like ride time, distance, cadence, heart rate, etc.) There are some bar graphs and pace arrows which may be an attempt on the iBike to supplement this, but they don't do much for me over the road. There is an interval mode approach available on the iBike, but it basically is a specified watt for a specified amount of time preset that you create via your PC before the ride. For me then this means that if I want to ride a ladder of intervals at a certain progression of watts, like on hill repeats or with negative split sprints between stop lights, I can't see a read out that shows me whether or not I am maintaining my watt targets, plus see the corresponding time component. The only approach that I can use is to toggle between the two basic read out screens during each interval. In a word, this blows. In two words, it psychotically blows. Try doing that holding your line in traffic at a full on clip and see if you like it any better than I did. And trying to do the press-and-hold stuff is even worse. Instead of a quick two button snap-shot that you can do by feel with the Powertap CPU, you instead have to do a hold and release approach by pinning down one section of five-way rocker switch to access the iBike's sub-trip capability.
Overall then, I can’t say that I would recommend the iBike Pro, either as a training tool or even on the basis of it being another geek toy for the more money than sense cycling set, of which I would definitely qualify as a member. I have seen on another forum one of iBike’s/Velocomp’s principals say that to date they have only had something like 2 units returned for a refund after the individual has taken the time to properly mount, calibrate and learn the unit. I have to say at this point that I will likely be amongst that number. They are cranking out firmware updates on an extremely frequent basis, the latest and version that I am currently riding is as of last Friday. I will likely give these such offerings a little more time over the road before deciding. As posts here and on other forums will indicate though, there are plenty of those who love these units and say that they are thrilled with them. So again, YMMV. Any differences in expectation and intended useage alone could well account for that.
Apologies to iBike/Velocomp and all if I have misrepresented anything or missed some key function that would have solved one of my issues. But given my uses, ride characteristics and perhaps any limitations as a sincere but knuckleheaded user, the iBike at its best will not be luring me away from my Powertap.
Now I have to go pee.