I recently - like, 2 days ago - purchased CS4 upgrade from CS.
First impressions are very favourable. It seems to run faster, and the Bridge function - which was a real trimmed down affair in CS - is much quicker too. It has some nice neat tweaks and interface changes, and a greater level of control with some of the filters etc. Very early days, so have hardly scratched the surface of the new version. Being 3 full versions on from CS, I'm guessing that there's a lot more to discover.
The best bit is Adobe Camera Raw, version 5.0. I recall using Camera Raw with an older version of Photoshop (v6) and enjoyed the fact that it's fully integrated. I think I need to get a good book on Adobe Camera Raw.
Any recommendations for ACR and CS4 books?