I am absolutely happy with my 5DIII, though I wouldn't say IQ is considerably improved (except in very low light). The big difference is the autofocus. I loved my 5DII, but got tired of using only the middle af point (the others were not satisfactory IMO). I remember thinking that my dream camera would be a 5DII with a 7D af, and the 5DIII has gone far beyond that. I am 100% satisfied.

I did consider the 1DX, but I really don't need it. I just wanted a full frame camera with a decent af, and the 5DIII more than satisfies.