I got a powertap. Best piece of equipment I bought for my bike. It takes a bit of reading and riding to get used to what the numbers mean but after that, you know the basics. I used my garmin HR strap as the powertap one is rank I reckon. Some people say 'I just want to make riding more fun and not about numbers all the time..' I say numbers are just numbers, its your choice if you let them decide if your not having fun or not..for me, its fun to train proper and avoid overtraining and get more out of my training..
Its great for improving your pedaling style, gear selection etc. As you can grind big gears and think your putting out heaps of power and then you put it in a 'wussy gear' and your actually putting out more watts now!
If I had to choose between a a 10kg road bike with a power meter vs a 6.8kg bike with no power meter,Id go the 10kg clunker for sure.
One of the reasons Lance and Cadel are so good is they starting training with power over a decade ago. Thats a lot of good data to use to see how the body is responding.
They are so cheap on line now..anyone in a western country can afford one. Ride to work for a few months and youve already paid it off!