Motorcycle specific GPS's are pretty nice to have if you have the money.
All are built to be vibration and weather proof and are glove friendly.
Some have blue tooth so you can pair up to various in/on helmet audio setups... like a Scala G4 Bluetooth module.
Some have the ability to accept an XM radio antenna or have it built in...
Some have the ability to receive weather radar maps... This can actually be nice when riding.
The question is... are they worth the price?
I personally use Ram mounts (
http://ram-mount.com/ ) to mount my GPS to my bike. I use a Garmin 1300t and have had it on the bike for 13 months and 11,000 miles. It still works just fine. I carry a few gallon size zip lock bags and rubber bands to hold the zip lock in place.
If you do the math... You can get a decent Garmin for about $150 to $175... each comes with a one time free map update. Even if you burn through a Garmin a year for a few years, you sill won't hit the $800+ price point of the Garmin 665 Zumo which has the XM antenna, BT, and weather receiver.
Now... Do I want the Zumo 665... HELL YES. Not having to worry about zip locks in the rain, and having the XM would be awesome. I have a Scala G4 for my helmet and currently stream tunes to my helmet from my Droid. ( I prefer a dedicated GPS to the Google Nav )... BT in the Garmin would allow me to stream the directions to my helmet, but to be honest, I don't need to. I just look down now and then and I rarely miss a turn.
Also...If you do mount one, make sure you mount it up and in your field of view. The ability to just move your eyes and not your whole head to look quickly at the GPS is far safer than looking down dead at your bars for info.