I think most of the guys use the Zumo 550. i'm sure they will chime in.
Yep, Zumo, but I want the 660. I don't need XM thanks to my iPhone. However, you will still need to get a speaker system. I don't know of any GPS that comes with speakers for the helmet.
I had the MotionXDrive on the iPhone...the $2.99 app...until I did a full restore to try to improve the ridiculously bad reception I've been experiencing over the last few months. Even though I synched the phone to iTunes first, and was able to recover every other app I had, this one disappeared.
It seemed to work pretty well for the money, but I hate to buy it again knowing it's not going to survive a system restore.
Usually at least in my experience with apps; if you select an app you already had it will let you download it again without paying a second time.