I have a Garmin Etrex Vista, and I've been pretty happy with it. It comes with a basemap of North American burned into ROM, but you can download detail maps to it. If you go with one of these, I'd definitely recommend getting one with as much RAM as possible, so that you have lots of room to download maps, track your trails, etc.
One drawback of all the units I've seen is the small screen. It's a tradeoff against size, and I definitely wouldn't want to lug a huge unit around, but I've always wanted a larger screen when I'm in the field.
After a few years, my Vista had some problems with the little navigation joystick and the screen developed some problems (probably my fault for driving with it on the dash of the car too much). I sent it in to Garmin and for something like $125 they refurbished it (replaced the joystick, new display, new buttons, calibrated, upgraded firmware, wash and wax, ...). I've been happy with it every since.