- Like what you've got: stick with it.
- Dislike what you've got: consider downgrading. That's really your only choice for D810 users. Even a D5 is a downgrade on the pixel side (though it does up the fps and body quality). A D610 or D750 is a downgrade in pixel count and body quality. Frankly, if you're in this category, you went too far with your "gotta have the latest" buying, and you need to take a good strong look at that problem before figuring out what to do next.
The question is whether D800/D800E owners should upgrade to a D810: if you’re filthy rich, sure, why not? The D810 does fix a few things and add some nice touches here and there over the D800/D800E. Many of us think the D810 is very much a different and better camera than the original D800, but that’s because of a lot of subtle and modest tweaks. From a pragmatic sense, there’s not enough there to justify the cost of switching at the moment. That said, I made the change myself, and the many small changes on the D810 make it handle much nicer than the D800/D800E, enough so that it almost—note I wrote almost—feels like a different camera.