I bought a used PowerTap (BDCheung's Old One) and it has been flawless. I've went through 1 set of batteries et all (hub, chest strap, computer) and I've been using it for 2800 km's (1700 miles) on top of however many miles BDCheung put on them so they shouldn't be a problem. Batteries were easy to find at a local store and breeze to replace.
For the wheel itself, if you go PowerTap I'd go with a bombproof rim. I have mine on a Velocity Deep-V and have had no truing troubles.
I have gotten the occasional drop out as mentioned, it happens about once, maybe twice a month, and is a 1-2 second blip where no data shows. It's really no big deal for me.