Well I don't feel I've used it enough to give a useful opinion, but early impressions are definitely good.
I used the D70 until mid 2010 when I got a D300s, but the D750 is still a significant upgrade, and more so than I was expecting.
There's a different weight distribution - the D750 body is lighter than the D300 and the lens is larger and heavier - but I think I prefer it this way.
One of the major features I'd seen mentioned is the fast and accurate auto focus, and whilst I've not yet figured out the details of the different auto focus modes (fortunately there's an auto mode selection option), it does seem to perform well there, so hopefully the number of lost shots due to missed focus will be greatly reduced.
High ISO performance is another hyped feature, and in limited tests so far it does appear to give useable images in the dark at high ISOs. (I think one of my mono shots will give an example, haven't got around to selecting them yet though.)
One minor annoyance is that the image preview zoom in/out buttons are reversed compared to the D300s. Looking at other models, it seems this has changed across the range from 2012 onwards. Probably more logical this way round, but still going to take a while to rewire my brain. (Although if I can trust the focus more, I wont need to zoom to check it as much.)
There's also the ability to tether a mobile and use that as a remote display/trigger - I've not yet used it so don't know if it's any good, but hopefully it will help when trying to photograph nieces and nephews, as well as some self-portrait ideas I have - don't worry, I wont be going all Candlish on you!