I've been wanting to train by power on my bike for a long time, and this winter I bought a Polar Power Sensor and used it on Lanzarote for the first time. It has been a huge disappointment, not picking up any signal from the bike sensors more than half of the time. Useless.
I knew it wasn't the most precise device in the market, the SRM crank arms, ergomo bottom bracket system and the PowerTap rear hubs are all much better, but they're also two to four times as expensive for a complete system.
Reading the Wattage list I've come to the conclusion that the Ergomo is the best, comparable to the SRM in repeatability and nearly as good in precision, if not the same with the latest firmware. Only thing is that the Ergomo only measure the left leg and then assume the right leg is just as strong, but it's within 2–3% in most cases on that account. And the repeatability is much more important.
There has been rumours about ergomo—or SG-Sensortechnik actually—redesigning the computer so the display will be flush with the handlebar instead of raised as it is now. It will probably still be in the ugly black/orange colors, guaranteed to look bad on any bike. I can easily live with a bad look though, as long as it works as well as it seems to do.