Whilst keeping the map as my primary means of nav, I have had a GPSIII for a few months and can't fault it (except my bungy fingers and the little buttons sometimes come into conflict). I have it sitting on the right hand side of the cowling and generally only ever use it to confirm my position or, more often, help work out ETA's and drift.
It has gone loopy on me a couple of times, once when coming back to a small field in the states with less than perfect vis and I knew it was pointing at least 30degress too far right. I ignored and got back safely. They're not perfect but they're superb in ridding the nagging doubts about my own navigation. I rarely fly without one now and the GPSIII is a nice compromise between functionality and battery life. I get 10 hrs or so from 4 AA's.
If you fly high wing, I'd recommend getting an aerial extension but haven't needed it in low wings. The only other accessory I ever use is the bean bag mount. Well worth the extra £10.