I have a Garmin eTrex and it's ok. Mine is an early one with serial connection, which is a pain, but I believe the new one are USB.

eTrex screen is small and a it's a bit of a pain to use sometimes. Even when mounted on the dashboard it's a bit tricky to see the screen. If I was choosing again I'd definitely choose a WAAS enabled one and go for a colour screen. The eTrex screen does claim to get to 5m, but is usually displaying more like 7m or 8m. Also the pick up in less-than-perfect-conditions is not very good.

Obviously, when working from 6 figure grid refs you're not that accurate anyway and if someone else has read those from a map badly then .... well, basically a GPS won't always lead you to a stone :-(

When looking for a portal tomb this may not be too much of a problem, but when it comes to rock art hunting all a GPS will do is get you into roughly the right part of the field on 6 figures.