My two cents, fwiw. I switched to Mac last September. Prior to that, I was running Audition 3.0, and planned to continue using Parallels on the Mac until I stumbled onto the Mac Beta version. I got that fairly early in the Beta process and offered what little feedback I could. I primarily use it for vocals and background music for VO's. Essentially, AA CS5.5 for Mac is what we Beta tested with the fixes. So having been part of that process, I am obviously stoked to be running it and on the Mac. No, I wasn't thrilled with converting files from ses format, but it wasn't bad and it translated well for me. I own the Production Premium suite as well, and was never impressed with Soundbooth, and still hate it. I was naturally glad to see that my Prod Prem upgrade would save me from upgrading two products.
Usability was transferrable for me. I was, by no means, doing anything earth shattering in my mixing and edits, but there was the little bit of change for me, not only from AA 3 to Beta but also to the Mac way of life. Maybe that made it easier, but who knows. I recently took part in a webinar for VO people who use AA and it covered features in CS5.5. I learned a few goodies that I didn't know existed and have cut some time off of my edits. I also do not use MIDI for much of anything, but I haven't had a need to try adding it in yet. I moved away from doing my own MIDI a long time ago.
My experience may not rival too many people here, but for someone who has only been working with Audition and for about 2-3 years, I can say that the upgrade is huge. I find it very easy to use and it makes me sound good. Take my opinion fwiw, but I like it and recommend it.