Ive got the Etrex20, use the pseudo OS mapping from talkytoaster
http://www.talkytoaster.info/ukmaps.htm#GPSRs-Only
loaded the one with contour maps and yes its just like the OS map but didnt cost £100. You can find maps for all over the globe, Ive saved one of france onto a microSD card for future Alps trips.
The etrex arent bike-specific so dont talk to HRMs, cadence sensors etc and also use a different Garmin software/site (Basecamp not Connect) so if youre into sharing stuff with mates who use Edges it wont work easily, but it is a brilliant bit of kit and the bar mount, whilst overpriced and basic, is rock solid.