I have a garmin 60csx that works great. They make bike mounts for it as well. Does everything you could ever want.

However, if you want to actually use it for directions, you'll want to upgrade the maps, which will tack on another $100 to the price of the unit.

I just use it for speed, elevation, time traveled, average time, etc. since I know where I'm usually going. You'll definitely want to keep a few extra sets of batteries since I think the rated time is something like 8-10 hours.

I've used it in the rain with no problems. The units did have a problem with vibration, which of course is multiplied on a bike vs. walking, however I had my original one replaced under warranty and haven't had any problems with the new unit.