I just bought a Garmin Nuvi 765T from Amazon and found a waterproof mount for it on Ebay.
Garmin Nuvi 755T 765T 775T Waterproof Motorcycle Mount | eBay
Soon as I get it and put it to use I'll post up my opinion on it.
Main reason I got this one is price and headphone jack. I've read conflicting posts about whether it will work with a bluetooth headset. If so, I'll try to pair it with my Scala headset. If not I'll use earbuds and the jack. I wish it had better battery life. My little bike doesn't have a lot of power to spare. If I wire in a charging outlet I'll have to switch off my light bar to make sure I don't over tax my battery when the GPS is charging.
Some friends had both a Garmin and TomTom and the Garmin seemed easier to use. I had an Etrex Legend that died on me earlier this year. Not a great GPS for driving but I'll be replacing it too since I like to hike.
I saw a TomTom Rider 2 on ebay for a good price but missed out on the auction. The Zumo series look good but were a whole lot more money than I was willing to pay.