I have the Garmin Nuvi 260w, and it works great... I think I picked it up for $149 at Costco last year... and really, as long as points me in the right direction with just a quick glance, which it does, that's all I need, and like idmtchris said, around town, you can hear it fine.
Took it with me to the Grand Canyon last year, and it went through rain, and 100+ temps with no problem. It sits behind the windshield, so no heavy rain got on it while riding. When I stopped, I simply unplugged it, un-velcroed it from the mount, and put it in my pocket. But a sandwich baggy with a rubber band would keep it dry if need be. It's been on the bike many, many times, even after the buzziness of the 900 at speed, it's still works great, and it does double duty in my truck
