It's a discontinued product from Garmin. According to their website, it's not motorcycle friendly which usually means not waterproof.
According to the owner's manual, page 60:
http://www8.garmin.com/manuals/StreetPilotIIIGPS_OwnersManual.pdfIt says the Streetpilot III is waterproof to IEC Standard 529 IPX7. It can withstand immersion in 1 meter of water for 30 minutes.
I've had mine on the bike for a couple of years and have been in a couple of downpours with no problems.
As for the usefulness of the unit, it fills my needs just fine. It does use proprietory data cards and the largest it will handle is 256 meg., which isn't large enough to do the whole US. But it does do a decent amount of states. It's probably not practical for a cross country trip unless you use multiple cards, but for a day or two of riding the storage is adequate.
The screen redraws are slow compared to some of the more modern units, but it does it's job.
Is this a new one, or is it used? You'll need to get a copy of MapSource to do the mapping stuff.
-Bob
