The D810 has had great reviews and I cannot see why one would not want one, I can see why any D800 owner would be tempted, but there is a £1,000 price difference between a used low clicks D800 and a UK retail price D810.
The D750 is another consideration, it's image falls between the D6xx and the D8xx ........ it may have a problem in that most will aspire to a D8xx or D4S
I think that I would be more tempted by a good used D800 or D810, (when available), rather than a new D750, but I mostly buy used
but we are faced with quite a few good choices, D700, D600, D610, D800, D810, D3S, D4 and D4S and now the D750 ...... they are all different