When the public beta of Audition 4.0 for Mac came out I downloaded it straight away and blogged about my first impressions. Since then I've enjoyed having a version of Audition on my MacBook, and now that the final release has come I've taken advantage of a generous education discount and bought the full version.
I was surprised to see the new version of Audition included in the CS suite, because I thought Soundbooth would stay there and Audition would be on it's own branch. However it does make much more sense to bring Audition in with the other pro apps. Soundbooth has been discontinued, so there's no budget audio editor from Adobe: it's Audition or nothing.
There are a couple of other things I've noticed that are different to Audition 3.0, and probably were in the beta too. In the multitrack view you used to be able to drag a clip out of the files panel and move it across to the right hand end of the timeline prompting everything to move left and allowing you to find the end of the previous clip. This was really neat for quickly putting together radio packages and doesn't work in the new version. I also liked the "Autoplay" tick box in the open/import audio file dialogue box that allowed you to preview files by clicking on them which was handy if you had a large directory full of sfx or other files to browse through.*
In the beta there was no "Generate" menu which in 3.0 allowed you to create tones and noise. This is still missing, as is the ability to group clips in the multitrack view.
For a full list of what hasn't made it through from previous versions into Audition CS5.5, or what's been renamed have a look at this page on the Adobe website.
Over the next few days and weeks I'll explore the package further. If you've questions, or things you want trying out (mainly from a broadcast production point of view, I'm not planning on recording an album) then leave a comment and I'll do what I can to help.