I haven't seen many first-hand user reports yet for any systems, so I thought I'd do some research.
I downloaded GAIAGPS for the Android, and am impressed with what I see so far. Good topo detail; forest road detail; real-time compass. It will probably suck the battery, so I'll have to either turn it off while not needed, or plug my Droid into bike power supply. Demo is free for those who want to test it. Full app is $9.99. I'll report back with more after I have a chance to get in the woods with it, but it's pretty impressive from here in my office at work (shhh!!).
First impression: This app appears to give the Droid user a viable, low-cost alternative to a "real" GPS.
More to come,