I have a Magellan Roadmate 2200 GPS I got a little over a year ago for $250. I think the best use for it is when you've wandered for quite a bit and are low on gas (or maybe hit your reserves, if your bike has one), but it has other good uses as well, as when you want to find food of any kind (Italian, Chinese, etc.) or lodging.
The 2200 is great 'cause it's waterproof to IPX-4 spec, which basically means you can ride in the rain with it, if you happen to be in such a situation.
However, riding with one of these suckers on your bike is, if you're not cognizant, an invitation to an accident. After I almost rode into X object for the third time trying to follow the map did I finally realize I better rethink how to use one of those suckers.
That aside, it's best, in my opinion, to use one as a last resort when you've had one hell of a great day and rode yourself into the middle of nowhere, and probably don't really have a good handle of where the hell you are. I mean, what's the point of riding by one of these things anyway? Kinda takes away some of the reason we ride, no?
If you plan a ride well, getting that lost should rarely happen however. Personally I like working with a good printed map before I go out because you can not only get a big picture of what and where, but it places you in a better navigational head. I think it makes for better seat of the pants navigation if you've scoped out the lay of the land on a map. You can't do that on a GPS.
That said, there's no denying the benefit of having one on hand. Highly recommended. Unfortunately the 2200T is no longer sold and Magellan makes no more IPX-4 spec units. The only unit like that today is the overpriced Garmin Zumo series, although they're finally coming a bit more down to earth. I think that when it comes to getting a motorcycle GPS the only game in town is one that's waterproof, and right now unfortunately the Zumos' are the only game in town, save for a few stray 2200Ts like this one:
http://tinyurl.com/cjzd3b