what did you have before the D750 mike, the D7100? huge improvement then?
It depends on what kind of improvement you're looking for. The d7100 is a fantastic camera. The problem I was having is that when I needed high shutter speed, I did not want to go over 1600 ISO because of noise. BIF shots were good at that ISO, but I needed more shutter speed without opening up the aperture more. What's left? Higher ISO.
I don't even worry about ISO anymore, unless I'm getting into the 12,800 range. Even then it cleans up really nice without losing a lot of detail.
I have my auto ISO setting at 6400 now, where with the D7100 I had it at 1600.
About image quality now. At base ISO the D750 has a higher Dynamic Range but the D7100 is no slouch in comparison to the IQ department..