I liked the open-endedness of reaktor4, but hated the pain-in-ass to use aspect of it. It worked really pretty well as a stand-alone, but was shite in Logic...for me.
So I bought a Nord Modular G2 keyboard, and love it. The Nord has the same or better synth patch/edit/creation features and is loads more stable. It cost four times as much, but what the hell...