Your Garmin help, please

Anyone using an eTrex for navigation?

I can't find the info I want on the web and maybe someone can shed some light.. Does it come with maps? If yes, what kind of maps? If yes, what's the coverage of the pre-installed one? Can I install OSM maps like I do on my 800?

I am really tempted by the AA batteries for touring but I am not sure I can justify it over the 800 I already have.

Not really. They come with a basemap which isn't really a map. It's VERY limited.
I've not tried OSM on the eTrex but it looks possible :http://forums.mtbr.com/gps-hrm-bike-computer/how-can-i-get-openstreetmap-background-my-etrex-20-a-875417.html

If you already have an 800, why not get the extended battery pack or carry a charger/batt pack?