I have the Zumo 550 and am very happy with it.
Have you gone to
www.garmin.com and compared the specifications?
Apart from not getting a lot of the bike friendly software features, you don't get :-
1. Hard dedicated buttons.
2. The RAM bike mounting hardware set.
3. The bike mount with power supply (the car mount is not waterproof).
4. The touch screen is smaller and may not work as well with gloves.
5. No Bluetooth.
6. No headphone outlet, so voice guidance is not possible on the bike, see 5 also.
7. No MP3 facility which doesn't really matter, see 5 & 6.
8. No gaurantee it's proof against the vibartions, exposure to chemicals and UV on the bike.
You do get what you pay for! The 660 is $ 100 cheaper than the Zumo 550, you get a bigger screen, better software etc.
In fact, do you want to buy my 9 month old Zumo 550 and I'll have the Zumo 660