I have a GPS III+ and have been using it for 2.5 years. It goes with me on every back-country and long distance road excursion. It does have MGRS, not that I use it.
When I first got it, I did have to go buy a book to understand some of the more advanced concepts with GPS's, like the UTM grid. I use the UTM grid exclusively for my coordinate system.
The Garmin units are very powerful. They make great operating system software and are always improving it for each unit.
If you get a Garmin, get one with the base map built in and some memory to upload additional road details.
I recommend that you use the GPS in conjunction with some sort of CD software product. I use the Topo CDs. These have all the USGS maps scanned in for defined areas. I draw out the path I want to take. Have it make a GPS route from what I have drawn and then upload it into the GPS. I then print out on a color printer the map(s) with a 500 meter UTM grid overlay, and then tape together all the pages to make a nice custom Topographic map for my trip.
It is very good! If you want simple and cheap, get a low end eTrex. If you want something for back country and road use get a better eTrex or GPS III+. If you want something really really nice for road and back country, get the GPS IV.
Garmin is great.