The powertap (and any power thing) is more a way of measuring your output so your benefit from it is directly related to how you use it.
Power is more immediately measurable than heart rate - so when you do a big interval using a power meter, you can steady your workload more effectively than if you had a heart rate monitor. With a power meter, you can hit your target (say 400w) and hold it. With a heart rate monitor, it's much less precise, and you'll probably go too hard at the beginning. In some of my hardest but shortest efforts, my hr barely gets moving while I'm going well into the 1000 watt range - it might go from 100-145 bpm but man I'm fried after the effort. Your hr monitor won't reflect that effort too accurately - it thought you just made a small effort.
For me the power is fascinating, I enjoy trying to "peg the meter" in sprints or big pursuit type efforts, and I've learned that I work much harder than I think on climbs. I bought my PT in April, rode it from May till about 2 weeks ago, but due to lack of training (promote 6 week series of races, prep/sell house, move, wedding, and honeymoon in seven months this year) I didn't race well.
What the PowerTap did to me specifically is make me realize that I want to know my power and heartrate for an entire ride (i.e. download it to the computer). Since I have a lot of wheels which are not PowerTap-able, I bought an SRM. Actually I bought an SRM bike - it was just a bit more than the SRM, and it's all new - and I can sell off the PT, a bike, and a few things off the SRM bike to help pay for it).
summary:
1. I love knowing power vs heartrate, full ride info (not just avg, max for speed/cad/hr).
2. I think I can use power/heartrate data to train more effectively since I have more time.
3. Due to the expense of converting many wheels, I am changing to a crank based power meter.
4. PT is good if you have one or two wheelsets. If you have more, it may not be cost effective.
I wrote a couple things on PT:
http://sprinterdellacasa.blogspot.co...ertap-srm.html
http://sprinterdellacasa.blogspot.co...wertap-sl.html
http://sprinterdellacasa.blogspot.co...wertap-sl.html
The second link has some interesting criticism on my use of the PT.
cdr