Dave - I've been using a Magellan 1424 for a couple years now. No doubt about it - a GPS can be a useful device, particularily when trying to find a specific service , such as the nearest gas station or campground. Also, having true speed, elevation, direction, time to destination, etc., makes a trip more interesting.
I opted for a car-specific model because I didn't want to invest too heavily in something that could be easily ripped off (although I do usually disconnect and store the GPS, when stopped for extended times). Rain is no problem - a plastic baggie provides waterproofing.
For bike use, a headphone jack, or Bluetooth capability is absolutely essential - you won't be able to hear the commands above city speeds, even lower if you use a full-face helmet. I installed a jack in my unit, and use a single earbud when I use the device. Not a modification I would recommend, however. Too much hassle!
Also, look for a model that allows "paved highways only". My GPS doesn't have this option, and has often pointed me down "roads" only suitable for dunebuggys.
For mounting, I made up a custom holder for the GPS, with it's own sunshade (in direct sun, you will not be able to see the display, which is another reason why you have to be able to hear the commands) I have it mounted so that I can easily work the touch-screen while riding, although I try to keep this to a minimum.
Cheers!