I've only had the D810 for a few days, so by no means is this a conclusive review, however I can say that I'm totally blown away by it so far.
I've been using a D700 for the best part of 4 years now, it's still an awesome camera, but the D810 kicks its ass in pretty much every single way. The D810's insane pixel count is one thing, but the dynamic range, the way the thing handles and the killer autofocus performance really make the D810 one awesome bit of kit.
I've read a lot of reviews on the D800 and D810 and people always say that the extra resolution shows up any flaws, such as back focusing, and therefore AF micro adjustment is key. On my D700 I had to adjust each of my lenses significantly, however on the D810, even my 50mm 1.4 seems ludicrously sharp without any adjustment, the only exception to this is my 35 1.4 which after a little use did seem slightly off. The ISO performance seems pretty solid, plus it's pretty easy to compress any noise out as there is a serious amount of resolution to play with.
It's still very early days with this camera, but I think it's going to serve me well! It's certainly made my old D700 feel dated.