I upgraded from K9 to K10U earlier this year, and have been loving the Evolve stuff, Rise & Hit, Cuba, and the Scarbee Rickenbacker bass (all Kontakt instruments).
Reaktor is hugely powerful, but takes a lot of time to grasp, so mostly I use it for the excellent preset patch banks like Razor, Steampipe, Spark, Prism, Monark, Rhythmaker, etc. I'm sure Reaktor 6 is very nice, but I think I can wait for the upgrade sale to K11U, hopefully next summer.