So I got the PowerTap in April.
I had a Cycle Ops electronic trainer (measures power etc) before the PowerTap (PT) - made by the folks who make PT. I liked training with power and heart rate - it seemed the right way to ride. I found it both motivating and revealing. For example, sometimes I wasn't going as hard as I thought - other times, the opposite. I found that loud music was worth something like 50-75+ steady state watts, that being awake and motivated probably 50+ watts as well.
Yes, this means that I sometimes train at 90 or 100 watts average since I rarely hold 200 watts average for 30 minutes or more.
When the electronic trainer burnt up (it started smoking and wires melted and stuff) I decided I wanted a "portable" power meter. Having power on the trainer was fun but I couldn't relate it to real world performances. I selected the PT after a lot of thinking, a lot of budgeting, and finally a quick swipe of the credit card.
So does it help? Does knowing power help?
I suppose the answer really depends on what you want from it.
Yes it tells you power. It downloads easily onto a USB equipped computer. It graphs your speed, heart rate, torque, and power. You can select a period of time simply by clicking at the beginning and end of it and the program will tell you your stats for just that interval. Power readings are available as peak, 5 second best, 30 second best, 1 minute, 5 minute, 30 minute, and 120 minute bests.
I haven't taken advantage of it as a tool though. For me it's more like a gadget.
I don't train that much - my PT, which I got in April and which I've ridden both indoors and out, has 850 miles or so on the odometer - and that includes about 60-70% trainer rides. I do use the PT all the time except when I've ridden my tandem (maybe 50 miles on the latter).
But I don't use the PT to figure out my next interval or how much power to put out on the next hill. I've never set out with a wattage goal in mind - it's usually a heart rate target and I check to see if I'm going well or not based on the power relative to my heart rate.
I have used it to gauge how I'm doing in races and during hard efforts. My best 1 minute efforts on the electronic trainer were about 400 watts. In races, I found I could do 450 watts for a minute, once 500. A few days ago, after writing the start of this post and thinking about it, I realized I hadn't done a one minute effort in a long time - a couple years. I did two and managed to maintain 500+ watts for both.
I found this pretty cool since in races I haven't been able to hit such sustained 60 second power levels. So now I know that the races I did earlier this year weren't as much work as what I did last night, at least for a minute.
Without a power meter, there's no way I would have known this.
Speed and cadence data is hard to interpret - there are too many variables to be able to extrapolate data from these numbers. Wind, gearing, incline, and perhaps elevation all affect your speed and cadence.
Many people use heart rate monitors and heart rate is a bit more telling. However, in my big efforts, a heart rate monitor really wouldn't have helped at all. When I did my two 500+ watt one minute efforts, my heart rate never went over 163. I blow at 170-173 or so. It shows that although I may not feel like I'm fit, I have some fitness. Or, more realistically, that my efforts were too short to get my heart rate up. Either way, a heart rate monitor would have told me "nice try but no cigar".
The reality was that I went harder for that minute than I have in any race I've done with the PowerTap.
In that sense, a power meter is good.
It's obvious too that if the power meter doesn't work then it's not really useful. In that respect the PT is very good. I used it in during a thunderstorm that was pretty bad - bad enough to "red flag" a Pro/1/2/3 race - and had no problems with the hub or the head unit. I've used it in pretty cold weather too although no sand or snow or salt. And I've banged around my bike in my trunk, the basement, and on the trainer. No data drops that I know of, no problems with the head or the hub.
Setting up the thing wasn't that quick but it was reasonable. The odd thing are the zip ties - if the bike gets really hot they don't stay tied. I got my bike out of my car one day (I left it in there during a really hot day) and virtually all the ties were loose - and I used a lot of zip ties. It was sort of odd. It looked like some of those magical shoemaker elves undid all my zip ties overnight or something. I used pliers to re-zip them and they've been fine since.
I'd like to get another PT wheel so that I have one for training, one for racing. A whole setup is not that much so I'd buy a second unit. I'd use the second "head" on a different bike or put it on my fiancee's bike. The second hub would be a 24 hole so that I can lace a 24 hole deep dish rim on it. I'd revert my first PT hub back to a non-tubular training wheel once the second wheel is built.
Of course this gets the price of the PT up there, almost into the SRM category. Almost. I'd have to buy a third PT to hit SRM levels. If I'd gotten an SRM to start with, I wouldn't have had to rebuild any wheels or anything. I still think the PT is easier to handle financially since you really only have to get one wheel. And a lot of folks apparently train with the PT but race on "race" wheels.
The one time you can't use a PT is if you want to use a real disk wheel. Since I don't time trial a disk wheel is not an issue. If you're a TT god you'd need an Ergomo or SRM to be able to ride a disk while getting power output.
Overall I like the PT a lot. If you're thinking of getting a power measuring unit, the PT is pretty foolproof, compatible with both Shimano and Campy cassettes, build-able like a normal wheel, and sets up similarly to a cyclo computer with cadence and rear wheel speed pickup.
I like it. I guess there's not much more to say than that.