I have a Rock n Roll with the KK power meter. Check the KK website, but I'm pretty sure you'll need an iPhone 4s or later, or iPad 3 or later. They have the Bluetooth that works with the power meter. Earlier Bluetooth won't work.
The power meter is fairly accurate for average power. I also have a Powertap wheel that I've used along with the KK PM. The averages are very close. Peak power measurements can't be done well with the KK PM because there is some time lag for the drum to speed up. For example, you can be doing 300W, then stand up to sprint at 1,000W. The KK PM won't measure the immediate spike of power from your sprint like a Powertap or crank based PM will. But it's good for more steady state efforts. And it's good for power measurements if you have nothing else already. Fairly cheap.
A downside is having to calibrate frequently. Variances in tire pressure and the pressure on the drum will cause measurements to vary. When the PM was first released, the calibration sucked. But the app has been updated and works much better. At first, I could hardly get it calibrated. Now, it seldom is a problem. And calibration is easy.
The R n R is very stable for standing up to ride or sprint. I have the first model, and it's pretty big. About 48" at the widest point. And heavy. Get something else if you're also wanting to carry it to races to warm up. Definitely not the machine for race day.
Feel free to ask other questions.