I played around with the demo quite a bit, but ultimately didn't buy it. It is incredibly powerful in the way that you can build your own instruments. You can create a lot of great, unique sounds with it, but it requires a lot of time and knowledge to get there. Ultimately, I realized that I would be spending hours tweaking a sound and concluded that I'd rather focus my musical energies elsewhere. They do have a version that doesn't have the same editing capabilities, but I wasn't as interested in that.
The program is a processor hog. I was running it on a 1.2 GHz G4 Mac and would frequently overload the CPU with just one instrument.
It is an amazing program if you are into designing your own sounds and I could imagine it being wonderful if you do soundtracks, sound effects, or ambient type music. Otherwise, I think it is probably overkill for most people/applications.
The new Reaktor 5 is awesome. Any kind of synth sound you can imagine along with drum loops and sequences. There are so many factory presets available in each ensemble, you don't need to program your own. The thing I really like about Reaktor is the constant movement of sound within a patch, works great for layering behind strings, piano or guitar tracks.