Well... Motorcycle GPS's tend to have some but not always all of the following features:
- Glove friendly
- Left hand friendly
- weather proof
- vibration proof
- XM radio capability
- blue tooth for linking to phone/helmet
-Zumo 665 has a weather radar capability
-Zumos have swappable batteries.
Now.. I use a Garmin 1300t on my Spyder fitted via a ram mount to my CHAD brackets.
I can buy four 1300's for the price of the Garmin Zumo 665. I put a ziplock bag over the gps and mount when it rains. I've run it for about 11,000 miles and it is still working.
Now I would love the Zumo so I wouldn't have to worry about longer term vibration and weather... and I really want the XM radio... But the price is too steep for me at the moment, and at least if I have to buy another cheap Garmin, I get a free map update.
I will point out however, that if you buy a really cheap GPS you will find various reasons to hate it. I think $150-$175 today buys you a good balance between affordability and features you may want + screen size.
I've found that Magellan makes utter crap for GPS's and their support sucks since they were bought by another company. Too bad as I thought they had a better user interface/GUI.
I've not used a Tom Tom that I can say I honestly liked...
Garmins seem to hold up well, and I've been really happy with mine.