I can't give any input yet on the Garmin Zumo 660LM, but I just ordered one from Amazon for $309. I've grown tired of struggling with car GPS units that never seem to stay connected to power as I ride. The cable going into the car GPS units always comes loose on me, or the 12v cig adapters lose contact. I've been using smart phones the last 1-2 years, but I have the same issue, compounded when I lose signal in remote areas. Of course, then there is the rain factor. I take a couple of longer trips each year where 10-12 hours per day on the bike are typical and every time I regret that I didn't purchase a true motorcycle GPS.
I've put off taking the leap for too many years, so I finally pulled the trigger. Although the 660LM is a discontinued unit, there are still new units, complete with mc mounts, wiring and hardware, available with standard factory warranties. Since they are not the latest and greatest model, they come with a more palatable price of around $300. It has the features I want and need. The only feature it doesn't have that I would have liked is the "curvy road" routing option. Outside of that, I think I'll be happy with it. It fits my needs and my means.

-Jeff