The one thing that makes me hesitate here is that if it's a Chaos Marine book rather than a Chaos book, I'm betting even more of the rule system will be rehashed. The main thing that bummed me out about DW is the RP section and character background and motivations portion was pretty light. It's hard enough to RP out a bio-engineered super soldier, it's even harder (for some of us) to RP out someone willing to give everything up to Chaos. I'd love to see a deep look at the inner workings and motivations of Chaos, I'm just fearful they'll have trouble delivering.
@Uncertain: It's nice to see someone else who can rationally view 'cost per hour of fun' - I fully agree with you. Though perceived value at the time of sale is still an issue I get stuck on. To me, the $60 core book, despite my logical brain saying "you'll get more out of this than that terrible movie that just came out...and it's even cheaper when you remember how much you just paid to fill your gas tank this week" still causes pain when I plunk the cash down on the counter