Hi,
I've tried a couple of eTrexs and several magellans out climbing. After losing 1 eTrax and destroying another (dropped down a cliff) I got a cheap magellan 300 instead. Despite it being an older design I actually prefer it; easier to use with gloves on, more basic. Gives you just lat / long or OSGB reference of your current position and can point you to another place if you want. Also pretty bombproof (touch wood) and batteries last about 20 hours constant use.
For flying though I wouldnt consider either particularly suitable. I used to take the eTrex up as a kind of backup, but after attempting to program it whilst airborne in a 22 one day I gave up and stuck with the skymap instead.
Maybe some of the bigger moving map units or PDAs could be better?