I've used multiple automobile GPS units on my FJR. Right now I'm using a Garmin 720 with a Ram mount - works great, but I take it off during the rain.
If you talk to garmin, they'll tell you that using anything but a "motorcycle" gps will void the warranty of a non motorcycle gps. . According to them, the other "non motorcycle" gps units cannot withstand the vibration from a motorcyle. Of course reality tell us something much different. I just bought a 765t thats has about everything you would want in a gps unit. Most of the forum members that have ever responded to my GPS questions use some sort of plastic bag for protection throughout the wet season without any issues with units that arent waterproof. This seems to be the big contraversy (waterproof vs. non waterproof (vs. vibration vs. non vibration). Many of us just arent willing to kick in the extra bucks for a non waterproof unit. Those of us that have, swear by them. I'm giving the cheaper alternative a go round before stepping up to the plate and paying the big buck.
I"ve got a tank bag that holds my gps and has a rain cover. I plan to use my gps when i tour (which is when i need a tank bag). i would take a good look at the application you plan on using. If you going to be using your gps daily, it may be worthwhile to go with the "gold standard". If its going to be used for "occasional" applications, the extra money could be put towards some other nice farkle IMHO.
hope this helps,,
GreyGoose