Personally I have been using the Tyredog for the past 2+ years, and have been happy with them. Yes, they are stem mounted, but haven't had any problems with rims being scratched in that time.
Tyre pressure seems accurate when compared to a digital tyre pressure gauge, also monitors temperature, not sure how accurate, but my concern is the temperature delta, not that actual temperature itself. I looked at a 5 or 6 transmitter unit, but figured the mucking around to change sensor position in software wasn't worth the hassle.
It has been extremely useful, as had two sidewall cuts on a previous trip, which the system caught before rim damage, and also caught a sidewall failure (all Cooper Zeons) which caused a slow leak. Also meant I could monitor the leak, and pump up when necessary, until I could get to flat stable ground to change the tyre.

