i use a garmin oregon, and the burns moto rectangular USB plug. I do not have that fancy little sleeve thingy to cover the plug end and make it waterproof. I did mount it under my gauges and behind the windshield, and thus far it and my unprotected end fitting to the garmin have been working without problems, even through some heavy down pours.
I actually just mounted a second one, to power both my droid and the garmin at the same time. seeing as the garmin doesn't stream music, and the droid tends to lose coverage when up in VT... I did have to replace the cable for the droid, I think the vibration got to the end into the phone, and it wouldn't keep good contact.