There are issues with both models, even now, not to mention GUI issues with the 660.
However for the most part the 660 sort-of works, subject to periodic spontaneous shut-downs, stuttering in the display updating, some very serious lacks in the Bluetooth implementation and instability in the device itself. The 550 has a well known issue whereby it sometimes fails to start up, necessitating removal & reinsertion of the battery from time to time (I believe it is temperature related and manifests itself most when it is cold out). For the 550 you should carry a small Allen key for this reason.
My personal choice these days is still the 276c and it's bretheren for it's far superior display, though for some the lack of a touchscreen makes it a non-starter. The main alternative might well be to buy a 765T and put it into a waterproof housing (if I wanted the onward Bluetooth A2DP connection) or better yet, a 760 if I didn't. The 765 uses the same crappy Motorola Bluetooth implementation as the 660, but it costs $400 less.
To quote one of my best friends, who happens to be one of Garmin's largest distributors when he responded to my question about what I should update the 760 in my car with; "Keep what you have. The last good units were the Nuvi 7X0s - it's all been downhill since then". Then again he has Kenwood in two of his cars and a 3790 in the third. His bikes have 660s.
I recommend you to visit forum www.zumoforums.com. It is a great platform to learn everything about Zumo's. I have Z660 - yes there were a few problems in the past with certain firmware version (3.8 was terrible), but from 3.9 (now with 4.0) works great and without any issues. I would not recommend to go to Z550 - it is older model and will be not supported and developed more. Some US users prefer Z550 because of XM functionalities (not valid for me, because in Europe this is not available) - for those there is Z665 available.
I'm not a fan of Zumoforums - when the 660 first came out I was seriously chastised for having had the nerve to suggest the 660 was buggy and I was beat up by some of their members who were outright lying about their experience and even about their 'connections'. When caught out, the owners banned me.- then, one day I was 'attacked' by the owner of the site in posts on poi-factory.com.
People at Zumoforums seem to have finally figured out that the 660/665 sucks in a number of ways and they no longer gang up on dissenters (and if you think 3.8 was 'bad', you should have been around for 2.7). However they don't have the maturity to accept criticism of their beloved devices with maturity.
My REAL problem is that the 660 still sucks in key areas more than a a year and a half after release - it is NOT good. It is simply better than it was. For what it costs and the time they've had it should be perfect.