I own the D7100 and am totally in love with it, The D7100 is actually a camera with very few weeaknesses.
Its main weakness is the size of the buffer.
For the general user its AF system is really more then enough, its a super sharp camera (with the right lenses of course) which does very well in low light for a crop sensor camera, the DR onit just just amazing.
It has enough control, knobs, features for anyone that isn't a pro and even for pro's this could be a wonderful second body camera.
I see my future in FX, I upgraded all my lenses to work on FX and I am mentally ready for the jump but yes even the D750 which no doubt is a great camera doesn't makes sense to me to upgrade to. If money wasn't an issue then I would jump on it or probably the D810 but money is an issue and I need to calmly think what will the D750 do much better for me to sell my beloved D7100 to get it and honestly I cant think of one think that will justify such a large investment when I already have a superb, wonderful camera like the D7100
Again at the end of the day its a matter of money, isn't it always ?
If money wasn't an issue I think most of us would own the D810 right now.
I think Nikon made a very big mistake when they made the D7100, it really is such a good camera for the price that it keeps many Nikon users from going up to FX body.