My 3880 died recently and I replaced it with a P800. My monochrome prints on matte paper with the new inkset are indeed noticeably richer in the dark tones; I don't see much difference in prints on glossy paper. So far I haven't done much color with it, though. Also, Aardenburg Imaging's initial test results from prints made with the new inkset in a P600 suggest that Epson's claims about increased image stability are mostly valid. In all, I'd consider the new P-series printers a modest but welcome step forward.
The P800 isn't much bigger than the P600, and at various times with Epson's different rebates it hasn't been that much more expensive to buy, so it's worth taking a close look at the technical and cost tradeoffs between the P600 and P800 before making a decision.