This is one app any music nerd should have just because of it's sheer funkiness. Really you could doing "everything" with this package (once you pick up a sequencing module from the user forum) but it has it's strong points: 1) Granulators: simply amazing if you're a soundscape person. Probably worth the price alone. 2) Beat Creators: so many ways to funk up sounds into the new mad rhythms of the future. 3) FX: an incredible selection.
The weakest point of Reaktor is, and I know there will be some gasps, it's synths. Unfortunetly they all have a certain tone to them which is just, well, not so pretty. And some of the more interesting ones like Carbon/Kaleidon are so huge to look at onscreen that it's hard to tweak them.
You can find some goodies at the user forum. As in the SynthEdit crowd there are about 15-20 really active contributors. The forum lists 1500 instruments for download but many of those 4-5 upgraded versions of the same instrument, (some won't even work with the latest RS4). Realistically there are about 300 usable instruments in there but that's still pretty amazing. There is also the Amphitryon & Studio from lost-online.com which take Reaktor into a full workstation & sequencer environment for a very nominal cost... making this perhaps the only application you'll ever need. James Walker-Hall built those 2 environments and has contributed several instruments you will find in the RS boxed package as well.
For RS + the 2xtra apps I mention you'll be looking at approx $250+- for the most powerful music making environment currently known. With the ability to combine instruments, effects and live performance tools it is the first stop in your computer music dreams.