I recently bought the Zumo 590LM to replace my abused Zumo 660. It is really nice.
I prefer motorcycle GPSs to using a smartphone, for overall durability (waterproof, gasoline proof, survives 4+ years of direct scorching sunlight), ease of use while riding (like functional with gloved fingers).
If you have the money and you're in the market, I strongly recommend the 590LM.
The only thing it doesn't have when compared to the 660 is an Audible audiobook reader, which happens to be my #2 use (second only to street routing). I've invested several hundred dollars into Audible DRM'd books for my 660, so this was a bit of a tosser. I solved this gap with
http://www.aimersoft.com/drm-media-converter.html and an MP3 splitter app (to break my 24+ hour audiobook rips into 5 minute chunks).
Of particular note is the 590LM's integration with my cell phone; Garmin released a Smartphone Link app that links to the GPS over bluetooth, and feeds it traffic, construction, traffic cam, weather, and weather alerts, and feeds that into your routing.
Also of note is new-to-Garmin "curvy route optimization". I can't say it'll magically find twisty roads in Ontario where there aren't any, but I have had it automatically route me up Snake Road and Southwood 13 when I was in those areas.
I've been using the 590LM for a couple weeks, and I'm very very pleased.
As for mounting options, that's pretty bike-specific. The 590LM ships with both car (windshield suction cup) and motorcycle handlebar mounts (which is _very_ bike specific, and not usable on my K1300S). I bought a K1300S-specific steering-tube cap with a 1" RAM-compatible ball, and that's working very well.