As a longtime Kontakt user I finally broke down and got the new Gigastudio to see what I was missing...apparently a lot!
It's been so much more stable in ReWire mode with Nuendo 3 than the Kontakt VSTi. At least for large Piano samples with long releases and sustain. It finally sounds and feels like a real world class piano without requiring me to add judicious amounts of EQ and signal processing (OK, I did patch it through my SLAM, but even still...).
So I'm sorry your experiences have been so problematic or disappointing (to be fair, my install choked a couple of times on plugins). I can attest to being at least one user who's had a positive encounter with the new version.