Measuring power output from the rider on a bike is the ultimate training metric. Using heartrate is a distant second, and using average speed is just obviously ridiculous.
At the moment I only have a Polar CS200cad and a S625X for training, the S625X for logging data and the CS200 for cycle-computer functions and an extra conveniently placed heartrate display. I also have a Polar Power Sensor, but it was a huge disappointment, and I stopped using it after a week. So I really need some sort of powermeter that works, because heartrate only tell me how fast my heart is beating, and that is only interesting to know when compared to exactly how hard I really work, which can only be a guesstimate without a powermeter.
A powermeter would take all the guesswork out of it and make it an exact science, and it would make it possible to compare workouts on the turbotrainer with real rides outside. This is especially important as the heartrate tends to be at least 5–10 bpm higher—and monotonously rising over time—on the turbotrainer because of the lack of cooling from the wind.
Besides, I do my hard workouts at threshold at rougly the same average heartrate all the time, but how much power I generate at that heartrate should be increasing, but I really don't know if it is. I can't know without a powermeter.
So it was great when I found a test of powermeters from Tour. They conclude that for serious use it's either Ergomo or SRM, which is completely unsurprising. PowerTap is a very good product too, but I find the need for a special rear hub to be a pretty bad idea, as I'd like to get some serious race wheels some day, and you just can't get a PowerTap enabled disc wheel…
They also compare the included software, but I think that's kind of unfair, because everybody should just use CyclingPeaks WKO+ for analysis and be done with it. Really. And it's actually a special old version of that software that come bundled with the Ergomo, the new WKO+ version can also talk directly to both the Ergomo unit and a host of other devices.
So now I need to decide which one I want (and can afford), the Ergomo is cheapest, have the most features and memory, but is less accurate than the SRM. Both are about the same precision. The SRM can be accurately calibrated using weights and some calculations, the Ergomo is supposed to just be automatically correct (!), and that's the main thing the SRM has going for it.