If you get in the full format, then you should be clear in principle on the follow-up costs. The full-frame Nikon cameras are much more demanding when it comes to the selection of lenses. Who switch, will find that there are few affordable lenses for full frame. Anyone who believes that he automatically shoots better images with a full-size, completely wrong. There are umpteen tests on the internet that compare the technical capabilities of sensors with each other and provide theoretical readings. In practice, under normal light conditions but that is irrelevant. Without anyone dealing intensively with the camera and the technical possibilities, he is - get the D7100 and 16-85 to yield better results than with the D750 and a 24-85 and not exploit the possibilities of the full format - under normal conditions. Interestingly, it is only when you want to photograph at dusk or in low light (keyword "Availible Light Photography"). Here the larger sensors of FX cameras their advantages. But again, you have bigger venture ISO 3200 to determine differences already in the area. In this respect, the D7100 is sufficient for normal daylight photographers completely out, with the pleasant side effect that his camera equipment is half as expensive. In addition, the DX photographer will appreciate its "larger" telephoto range if it is animal or wildlife photographer. Of course you can operate a camera FX in DX mode with DX lenses. But it does not buy you this camera yes.
As for the service, so I would classify the D750 at the level of the D7100. Advantage of the D750 is the swiveling monitor (especially for videographers interesting) and the built-in wireless LAN function. This is interesting more for studio photographers who want to transfer the data to the tablet or notebook. For data transfer of large amounts of images you no good in my opinion. For this you can buy better a reasonable USB3 card reader.
Compared to the D800 it borders on the one hand by the lower resolution from (the D800 is really a pixel monster, but what it shows on the size of the files), on the other hand. By the operator, which is rather on "consumer" level For me personally, it does not matter, I like both operating concepts. Because you can configure the buttons on the D750 individually, I do not see this as a disadvantage. It's more a matter of taste. A positive attitude towards the D800 is the noticeably lower weight. One might almost think, you have the D7100 in hand. This has a positive effect especially on trips. In general I agree with the general opinion that the D750 is very well in hand.
For me, the D750 perfectly the gap between the smaller high-end camera and the larger professional models. It is the perfect introduction to full-frame, but at the same time meet a wide range of users. An amateur who necessarily wants to enter into the full frame, can be found on this camera quite at home, on the other hand it also provides for the serious amateur or the price-conscious professional enough potential. She is a full-frame sensor and D7100 with some improvements. A successor to the D700 it is not due to the operating concept. Unfortunately I can not compare with the D600 / D610 it because I myself have not had the two cameras in hand.
Conclusion: In this segment, I give the camera the maximum rating of 5 stars. Weaknesses I could not notice until now.
PS. Another addendum. Recently -problem is on the Internet about the so-called "flare" reported. This is to shadowing due to backlighting. Also, my camera is concerned, according to Nikon side of it. I tried to reproduce the problem and compared with other cameras. Yes, you can see these shadows. However, other cameras have shown this effect, albeit less pronounced. However, I had to provoke a LED light the situation. In my opinion, these are an absolute limit situation that does not matter for 99.9% of the shots. Therefore, I see the issue of overvalued as completely.
