Probably one of the best pieces of audio software I own, it's a vast and daunting beast. I really wish I had the time to sit down and learn how to build synths in it from scratch, but the manual ain't that good and very few beginner tutorials are available. But even if you only get as far as leaning how to wire existing synths and effects together, it's worth it for the included instrument alone, and then you have access to literally thousands of synths, sequencers, sample editors, drum machines and effects in the Reaktor User Library.
But that manual...gah, it's at least a couple of versions out of date so very frustrating to read.