NWN2 is worth the purchase, especially since you can pick up the NWN2+MotB Gold Edition for relatively cheap. It doesn't have the top notch story that BG2 or PST had, but its still entertaining to play through, and I really enjoyed aspects of it.
However, even with the most recent patches, the game is till poorly optimized and demands a lot of computing power to run effectively. Most of the benchmarks you'll see are flat out wrong though. My older 2900XT would perform almost twice as fast in NWN2 at the same settings as some review sites were getting with 8800GTXs. Maybe the game was just friendlier to ATI hardware, I don't know.
Storms of Zehir is due out later this fall, as is Ossian Studios' much delayed adventure pack Mysteries of Westgate.