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IME they're good for pretty much ever. The NDS bearing has a habit of dying relatively frequently, as it's rather small due to having the electronics surrounding it and it's not very well sealed. However, it's very easy to replace yourself and the scare stories about the hub needing to be recalibrated afterwards aren't to be believed, I've found. I've found Power2Max and PowerTap meters to be extremely reliable. Everything else has been a bit hit and miss, except for Stages, which was absolute shite from day one.
I've been considering getting a powertap hub/wheelset and have noticed that servicing seems to get mentioned quite a bit more than on other powermeters... Is this something that needs to be considered when purchasing one or are they generally good for few years?