It's an interesting combination of a few different signal processing tools - a transistor/tape/tube saturator, some dynamics control, a nice sounding soft distortion, an LFO, and a couple other small things. The basic presets are all really awesome sounding - I particularly like the tape saturation for most things, the dynamic punch presets for drums, and the tube distortion presets as a drop-in replacement for a TubeScreamer on my guitar signal chain.
Well worth the money. If I use all the activation keys that I get (I believe you get 3, but it might be 4 activations per purchase), I'm buying it again in a heartbeat. Works great on Linux.