Motorcycle GPS models are just more expensive, water resisitant, more fault tolerant touch pad etc. BUT I find the "inexpensive" TomTom One basic model works okay, not perfect, but does the deed for round $120 or so. Yes it would be nice to have a brighter display, and louder speech...but on the plus side, it is MUCH easier to use than either Garmin or Magellans, the Zumo in particular seemed to display things like an old Nintendo. The TomTom I especially like being able to view the route in text or program alternate routes quickly.
Best endorsement yet is my gal picked up using it the very first day I showed her, and now she pulls it off my bike all the time to run in the car going on trips or garage sales on weekends. I like technology, actually a techno junkie, but she likes using them as the tools they are made for, so if it is easy to use and transfers from vehicle to vehicle quickly it is a wrap.