Fitting between the D610 and D810, the D750 combines the superior autofocus system from the D810 and D4S with the lower resolution 24 megapixel sensor of the D610. If you plan to take any sports or wildlife shots, it’s a worthy upgrade from the D610. However, if you’re a travel and landscape photographer, the D610 can save you $500 that might be better spent on lenses. 
As the middle child in the Nikon FX lineup, the D750 is mostly made from parts of other cameras, but there is one very unique and important feature: a tilt screen. While dozens of cameras have a similar tilt screen, the D750 is the only full-frame camera to have one. They’re immensely useful for creative composition, including shooting still subjects slow to the ground or when holding your camera over your head (for example, when shooting in crowds). However, the live view autofocusing is rather slow, so don’t expect to chase wildlife or football players using the tilt screen. If you plan to buy sharp, high quality lenses, upgrading to the D810 can provide you with more detailed images. However, if you choose to use the fairly unsharp 24-120mm f/4 kit lens, the D750 gives about as much detail as you’ll be able to get. Existing owners can use this free hands-on tutorial: https://www.youtube.com/watch?v=7RUwyyK9vss
