I have never used a GPS before but would like to get one to calculate
elevation, grades etc especially on hilly routes. My cycle computer
already has speed distance etc. The Garmin Etrex seems to get the most
positive reports. But that's from the general public. How about bikers?

I'm not too interested in preprogrammed routes (though I probably will
after I get it). More interested in downloading data to a computer
after a ride to get a visual display of what I have done. Will it do
this? I expect it must update your position extremely frequently or
even constantly otherwise a slow climb followed by a fast descent would
give a strange readout.

Possible with the GPS or with add-on software?

Mountable on a bike? Can you just carry it in your pocket?

I read someone's post about how he had to edit his route after
downloading as it included his car ride home! I would expect it to have
some kind of mark feature to mark beginning and ends of routes.

Since I'm brand new to these things any advice is welcome. Thanks.