Fieldsher,
I bought my wife a c330 to use (c340 wasn't out yet) for her vehicle, since it was less intimidating to use than the 26XX or 27xx series Garmin. She likes its ease of use and compactness.
As for use on a motorcycle however, besides the lack of environmental protection on the unit , I would add one more objection which I would consider a serious flaw in such an application.
The screen on the unit is resonable bright in a vehicle as long as you are not driving with the sun in front of you, although, it's not as bright as the 26xx or 27xx series screens. The real problem becomes apparent if you put on a pair sunglasses, whereupon the screen becomes almost totally opaque, particularily if your shades are polarized.
I would recommend a 2610 series with a +2 Gig memory card, their cost has been reduced considerably, or if you have the resources, a 2720 which has the latest capabilities. They are usuable on a bike or car and are weatherproof, and their shape leads to easier mounting options. You can also buy some additional Garmin dash mounts to transfer it quickly from one vehicle to another.
BTW
My wife quickly learned to use all of the Garmin units, after having her initiation with the c330.