I had the 305, now have the 500. My fiance has the 705.

The 500 has an altimeter and supposedly measures vertical climbing speed (though I have no idea of its accuracy at doing this)

You can program workouts into the 500 just like the 305.

You can also program in courses (ie: maps, but without the actual road names). I'll map a route using Bikeroutetoaster.com and upload to the 500. It beeps when I need to turn and tells me which direction to go. There's no actual map, but I've never gotten lost using the 500 this way.

I also like the 500 becuase it's a little more streamlined than the bulkier 305 and 705. It's easy to program and seems to have everything I need. My only gripe is that I haven't figured out how to display maximum speed on one of the three display panels. Anyone know if I can or if Garmin screwed this up? I can download it after the ride, just can't see it during.