I use the Etrex 20 myself, and I leave it turned off except at intersections and forks in the road. A pair of lithium batteries lasted me two months of my recent tour in the Mojave desert. The main thing is to make sure I turn the GPS off after referring to it. On a previous tour, I several times left the GPS turned on all night, so that it ran through several sets of batteries before I reformed my habits.
The Etrex 20 here in the US is available as a bundle with Garmin 1:100K topo maps for the entire United States for under $300 at REI, or a lot less than that if you get the 10% member discount and shop at the 20% off sales. A very good deal. The Garmin 100K maps have lots of errors with respect to the dirt road network of the western US, so it's best to use in conjunction with the Benchmark road atlas paper maps.
As for the many "features" of the Etrex, my opinion is that you should simply ignore all of these so-called features. They are clutter, just like the "features" of most computer programs. The great innovation of GPS was to tell you where you are on a map, versus the old system of triangulation with compass, sextants, etc. You need one screens for that, the Satellite screen. Combine the map into the GPS device is also very useful, so you don't have to buy and carry around so many paper maps. You need another screen for that: the map screen. And of course you need the setup screens to choose coordinate system, etc. All the other "features" are of minor importance by comparison, though storing waypoints and uploading/downloading waypoint files from a computer can be useful.
Instead of spending time looking for and reading Garmin manuals, I advise learning about GPS. The concepts of ellipsoid, map datum, Molodensky transformation, coordinate systems, etc. The mark of a person who understands very little about GPS is to give UTM coordinates without specifying the map datum. This is less relevants in the UK, where the coordinate system is always British National Grid with Ordnance Survey datum. Whereas in the US we use both NAD 1927 and NAD 1983 (=WGS84) datums with UTM coordinates and in many parts of continental Europe they use both ED1950 and WGS84 datums. The wrong datum can easly put you 500meters off course. Greece uses map datum that isn't in the Garmin database (Greek Geodetic Reference System or GGRS) so you have to specify the Molodensky transformation for those maps explicitly.
