Training without a power meter is like going to a gym that has no weight listed on any of the free-weights or universal equipment, or running on a treadmill without the speed showing. You can still do a great workout based on feel, but knowing what the weight or speed is significantly helps with the incremental increases in your training plan.

I have about 10,000 miles on my PowerTap G3 Hub (connected via BlueTooth to a Garmin 520). It works flawlessly, and the single coin cell battery lasts about 1500 miles. I display 10 data fields on my Garmin, but Watts are the thing I look at 99.9% of the time. Everything else is mostly superfluous: speed, time, temperature, calories, distance, cadence, altitude.

I chose the rear hub power meter because my 3 main bikes all took the same size 700c wheel, and I wanted to be able to move the PM between those bikes (though I do have to swap the cassette if I'm putting it on either of the "old" 10 speed road, or commuter).

The crank arms on those bikes were not compatible, so Stages or 4iiii PM's were out of the question. And the pedal based PM's only came in SPD-SL (at least at the time I was buying), and I only do SPD shoes and pedals. But if SPD pedals existed, and were known to be equally reliable as the PowerTap hub, I probably would've went that route just for the sake of swapping convenience.