I only use about 5% of the CS3's vast feature set, but I'm an upgrade junky. So, I already pre-ordered PS CS4 and InDesign CS4 last week.
95% of my digital imaging needs are very well served by Lightroom 2.1. But, the closer integration of PS CS4 to LR is a great feature. The new layers capabilities will make it easier for hobbyists like me climb the learning curve.
As a member of NAPP, I already reviewed the PS CS4 tutorials on the NAPP members web site. It really looks awesome.