I have been a GPS user for years. Essentially, if you know the area, they really arent gonna do you any good. And, it will only give you your coords, which you will have to compute into whatever datum your map is (most GPS units have multiple datums, just ensure that it is set up for the topo you are using). It is a tool for quickly figuring out where you are IN CONJUNCTION WITH your map/compass. Again, on familiar territory, this is largely useless.
The higher end ones allow you to load a map. I have a Garmin GPS 60Cx, which I loaded topos for the whole of new england onto a memory card. With this, I can find a place I havent been to, and, in theory, hike using only the GPS. downside is that the topo programs I have to show alot of trail detail. Of course, I would have a map with me regardless. Topos have alot more info on them than the GPS can hold, as well as giving you a much larger viewing area.
If you are looking for a basic, no frills GPS, I suggest a garmin etrex (yellow), or, for a little more money, the garmin 201, which you can attach to your wrist like a watch. Neither of them are capable of supporting map software, but, having a map with you, you can easily plot your grid.
Hope this helped a little.