I'm a reaktor 4 user - it is amazing, and i hear reaktor 5 is better still. Really good reverbs and echos and synths out of the box and once you've got the hang of building your own you can build pretty much anything at all...
If you figure out how to temp sync stuff automajikally let me know *begs* I could never get my head around the maths and it stopped my producing an amaaaaazing synth which revolved around sine (or whatever) waves tuned to the harmonic series which automatically varied in volume over time and/or phase start point, each sine wave also allowed phase start point control over the LFO modulation it received meaning you could 'mexican wave' through the harmonic series. You could then treat the whole thing as a single osc and sync it in various different ways to another of these mad oscillator things and then the signal got spat through filters amps choruses and delays.
It was utterly mental.
But I never could figure out how to tempo sync *sob*
Reaktor 5 is wicked for creating sound FX.. Just mess around with some of the granular/sample slicing type instruments for a few hours and record the results and you can build up a clollection of totally unique fx Not sure I can be arsed to actually make my own instruments though - why bother when you can get the reaktor user library? I know that if I start making my own instruments then progress on my tunes will stop completely!
Granular synthesis is Reaktor 5 is awesome, some of the mind boggling stuff you can create with a simple single hit drum sample or weird little effect is crazy. The sound generators are awesome also, have a play with some of them for some weird and wonderful sounds that can be chopped up for all sorts of madness.
Then you get to play with soem mind boggling synths that have some fantastic features and functionality.
The effects are betetr than most and can add all kinds of depth and space to your music.
Its all about experimentation and when you feel comfortable go make your own, then you really get into the power of Reaktor 5, and it gets frightening what you can do.