I know I'm a rookie HD rider, and still trying to get used to the prevelance of those who don't ride much in foul weather, but being able to get full use out of my GPS no matter what the weather has always been a must. I've made due with putting a waterproof cover over my radar detector, because it's not like you actually have to do anything with that, but being able to have full use of the GPS is critical. So I've always had a waterproof unit. If you're a fair weather rider who might get caught in the rain every now & again, having a non-waterproof unit with an emergency ziplock bag stashed would probably work and save you a few hundred bucks... unit you forget the ziplock and get stuck inan unexpected shower of course... lol
Don't care about XM and want stereo bluetooth, so I'm getting a 660.