Summary: As an owner of a fully expanded K25000S, I found the original PC3X to be impressive and frustrating in almost equal measure. By changing to a modern storage medium and (most importantly) by adding sample playback, Kurzweil have swept away many of my reservations, and I now like the PC3K8 a lot. Unfortunately, there's still no provision for sampling. Sample playback, yes. Sampling, no.
Pros: VAST synthesizers remain a pleasure to play, and they still sound superb., The reintroduction of user sample playback is a huge benefit., You can now import WAVs and AIFFs and build sounds upon these., Changing to USB storage is a major improvement.
Cons: It's time that Kurzweil adopted modern screens and GUIs., It would have been nice to see the KB3 organ emulation improved., The complexity of VAST remains daunting for many potential users., There's no sampling option.