ergonomically it was horrible in the air.
With only five buttons on the "Euro" model (later models have six I think) the eTrex will never be ergonomically friendly. Particularly not if you compare it to touchscreen devices like the iPad.
Despite having used (and liked) my eTrex for over 15 years, I think I have never ever entered a lat/long coordinate using those five buttons. Well, maybe once, as a test. The only waypoints I ever created in anger using those five buttons were "mark present location" waypoints. Similarly, the times that I created a route on the eTrex using those five-buttons and predefined waypoints are very rare.
What has proven to be useful is the "direct to" functionality, where you select a waypoint and head straight to it. And the track log of course.
But honestly, getting all these aviation waypoints in, and the routes, is something that's best left to OziExplorer. A bit of planning helps, but I have had my laptop on the wings more than once, to do a last-minute load of the route into the eTrex just before flight. In that respect, get one of the eTrex models that include a serial or USB cable by default. The Vista for instance.
I've been quite happy with my eTrex for flying for a number of years. But would I buy one today if the primary purpose is flying? Probably not. There are so many aviation-specific units available for not a lot more money that getting an eTrex for flying is just plain silly.
Other than that, it has proven itself to be an extremely rugged device. Apart from flying I have used it for hiking, geocaching, sailing and various other outdoor activities, all over the world. It has gotten soaking wet on a number of occasions, been dropped, banged about in the backpack and whatnot, and is still going strong. Two alkaline AA batteries will last for dozens of hours, and even with NiMH rechargeables the unit will be powered for 10 hours or more. And despite the iPad and the on-board GPS, it still lives in my flight bag as a backup.