I pulled the trigger on a new CycleOps Powertap G3 hub recently, and built it up with a new Mavic Open Pro CD rim. I've managed a few rides on it already, and am starting to make sense of all my wattage numbers. It's a rather expensive piece of equipment, but, if you're serious about training, power meters are the way to go. They give you a definitive set of data that point out how much work you've done, as well as the areas you can improve upon.
The G3 is lighter and better than last years model. It's only a weight penalty of 60 grams or so compared to my Chris King hub. It features internals that are more easily serviceable. Rather than sending in an entire wheel you only have to replace the end cap, which is where most of the circuitry is enclosed.
The more I learn about training, heart rate zones and wattage, the more I'm realizing how cycling as a sport is much more calculated and precise. Even aero bikes, bars and helmets can amount to wattage savings. It's kinda nuts. Speed and efficiency can almost be bought to a certain degree.
I'll be following up with a more in-depth review of the Powertap and the numbers it produces.