I have heard of issues with the Etrex series bike mount being not secure. I think I read about that on MTBR and some other cycling forums.

That would explain why a lot of people are making a DIY mount for thier Etrex instead of paying the $9-$15 for it because some have already paid that price and dropped (thankfully not lost) thier GPS. If you plan on using that Garmin bike mount make sure you use that lanyard attachment as a backup so if the GPS slips you won't have it bouncing off on the road/etc which perhaps a car could run over it or lose it.