I used an IBike Gen-3 for the last year. I recently added a PowerTap with a Garmin-Edge into the mix. The biggest issue for me was reliability of the IBike. This is totally subjective of course. I'm a very busy person and don't have time for problems. I have a short window of opportunity to get my rides in. When I get on the bike I just want to go and not be messing around with the power meter.

Here are my observations/ramblings:

1) IBike: Every once in a while I'd head out for a ride and the power numbers would seem off. This was corrected by doing their calibration procedure. The calibration process hads been much improved. It now consists of a tilt test (about 1 minute), wind cal (about 2 seconds) and a 2 mile ride (the time depends on how fast you go). After the calibration it worked fine. I do want to point out that I did not have to do this that often....maybe 5-6 times over the last year. I believe the IBike folks are aware that cyclists just want to get on the bike and go. It seemed to me that all their software upgrades were of the nature to make the power meter much quicker to use. I feel like they did do that. I also want to point out that even if they readings did not seem to be correct that I did not have a problem doing any of my workouts. If you know what wattage you're putting out on a climbing repeat and you notice its 50 watts higher on average you just use that number as your new target.

PowerTap: In the short period of time I've used the power tap I've had no problems. It literally is just get on and go.

2) The IBike folks do have good customer service. They are always responsive to any questions you have.

3) IBike: The embedded software in the IBike head is continuously being upgraded. Sometimes the software release has no issues. Sometimes the software has a bug in it. If any problems are encountered they do update the software. I do want to point out that I never noticed any issues with the power reading between software releases.

4) Concerning power readings...
IBike: The ONLY time I got strange power numbers was when it rained. I don't make a habit out of riding in the rain but when you live in Florida you are bound to get caught every now and then. It seemed to me that the aero sensor would get rain water into it and that's when the power numbers would do a prompt jump up and stay up. I'm talking in the 1000 watt range. It should be noted that once I got out of the rain and the thing dried out the power numbers returned to normal.
Power Tap: No issues with power numbers. Seem comparable to what the IBike was calculating.

5) My power numbers make sense on descents and when I stop pedaling. This goes for both the IBike and PowerTap. There is a slight delay between the calculation and when the screen is updated. I think that's normal.

6) IBike: The coast down procedure is gone and replaced with the calibration ride. I'm not sure if you can even do the coast downs any more. I've never noticed it in the menu system for the head end unit. When I first got the IBike you had to do the coast downs.....now you don't.
Power Tap: You don't have to do this. I put the wheel on and the Garmin found it. Off I went.

7) Ride analysis software. : I like the ride analysis software for the IBike and the Garmin Connect as well.

In general, both off them work with comparable power numbers. Ibike slightly less reliable. Just my 2 cents.