I don't know what everyone is going on about. The upgrade process is rather convoluted, but that is just because there is a very usable free trial that becomes the full version with an authorization code. I haven't bought 4.0 and don't know if it's possible to upgrade from 3, but at least I was able to upgrade the copy that came with Final Cut Pro 2 to the 3.2.1 version.

Very good option for basic sound editing needs. I used to use Amadeus, but numerous playback and conversion issues brought me back to Peak. Very fast and responsive interface, good support for the newest mac models. A couple stability problems but I haven't found anything better so far.