I use an e-trex as a back-up when I'm flying, and I find it quite handy. It works perfectly in an aircraft, the most useful aspects being the range to/from waypoints, and bearing to/from waypoints.
It's great when you're in an aircraft without DME and you want to report a range from, say, a VOR to ATC. If I'm in an aircraft that has full IR capability, I use it much less, but it does come in handy for finding VRP's that might otherwise be easily missed.
I fly in and out of Redhill, and being able to accurately judge where Gatwick's airspace begins is very useful.
You have to enter in your waypoints manually, which is great because you can choose the most useful one's for you.
One piece of advice though: be careful when entering waypoints using Lat/long, it can be very easy to misread the E or W part of the longitude, and therefore put your waypoint on the wrong side of the Greenwich meridian.