Reaktor is one of those products that just keeps getting better and better. When it first came out, it was ground breaking. Certainly there are other modular synths that have outdone Reaktor in one regard or another (SynC Modular and SynthEdit come to mind), but none encapsulate so much in one place.
I bought Reaktor Session instead of the full Reaktor simply because the full Reaktor is hideously overpriced. I love to program modular synths (I'm a Sync Modular fanatic), but I couldn't justify almost five bills for a piece of music software. At $150-$200, Reaktor Session is a much more reasonably priced alternative.
Although I can't create new instruments, the good news is that there are enough publicly available instruments that I may never need to! Further, while new instruments are not possible, combining existing instruments in new ways most certainly is. So a limited form of ensemble authoring is possible.
All that said, the UI is not particularly appealing, and is quite unintuitive in several areas.
What makes Reaktor Session a must-have is the user library. It is mind-boggling. There are so many ensembles, with so much innovation, that it's really hard to even put it into words. If you are an electronic musician of any stripe - be it ambient, prog elec, symphonic elec, techno, club, etc. - Reaktor is a must-have, for the simple reason that the library is so massive and powerful.
For what it's worth, I found the copy protection to be well implemented and not particularly intrusive. It did prompt me to re-insert my CD once, which was annoying, but overall I pretty much forget that it is copy protected. NI even allows for a duplicate license so I can take Reaktor with me on my laptop for business trips - perfect!
I *would* however like to make a backup copy of my CD, in case one of the kids decides to use it as a frisbee. Unfortunately, I can't. I wonder how much NI would charge if I needed a replacement?
In any event, Reaktor sounds *good*. Reaktor's not the fattest or the most authentic sounding soft synth, but it is still very good sounding. Combined with the massive user library, this is the synth to beat.