Ok. For starters, I actually have a GPS 72H. It's essentially the same beastie as any of the others in the Garmin range but you can't download maps (if you could, it'd be a GPSMAP see?)
We use it for round-the-buoys racing, by entering in the co-ordinates as waypoints using Mapsource and downloading over USB. If you knew the course in advance, you could setup a Route, but we usually don't so I just pick the next mark.
1. The 72H basically gives you actual, average and max (since turned on) Speed-over-ground in kts to one decimal place. The 'compass' heading is direction to next waypoint
and it's really easy to see at a glance if you're off course; you also get distance to next mark and ETA.
2. Although there is no map, the 'Track' it records is enough to show you if you are lifting or being knocked toward the next mark. It will tell you if you are off-track, but, no, it doesn't tell you which direction you're traveling; that's what a compass is for.
3. It will display depth (for the anchor alarm function) and speed-over-water with the appropriate NMEA sensors as suggested - but I think of those as being a 'future upgrade' and not essential.
I'm quite happy with it. It powers up quickly, is salt-resistant, tells me all I need to know and holds plenty of data, although Mapsource is awkward to use and I wish there were better software out there..
If there is anything specific that you'd like to know - ask away!
EDIT: I find that the 'compass' display is the most often used one since it shows current speed, time to next mark and - more importantly - the direction!
"Where's the mark?" "Over THAT way!!!"