I got my little Geko basically to do stuff like download a tracklog of where I'd been post-ride. It's a fun little device, but, referring to the points above:
1) I got the Geko because it's only 3 oz, smallest such unit I've seen.
2) Yes, I haven't seen cumulative elevation recorded on a GPS.
3) Batteries tend to last as little as five hours.
4) You definitely get this problem under tree cover. Haven't noticed problems with mountains/ hills.