Its nice to see History applied to a sound editor and the non-destructive effects, and its so far very stable. I tried the free Audacity, trying to perform simple functions made it crash.
As for features, its kind of nothing special, you can get more outa Garage Band which comes FREE with your Mac.
I find the interface somewhat confusing, especially the way how it allows for multiple sounds to be open at once. I would prefer each sound in its own window or to be able to see all the sounds at once.
I personally think that the interface controls (buttons, sliders, tabs and such) are hideous, reminds me of some bad haxie theme.
All in all the only advantage of SoundBooth is the history, but its only a matter of time till the other choices add this ability. I was excited to hear that this was in the spirit of SoundEdit 16, but let down by the confusing, cumbersome interface and lack of "features".