I strongly considered going to medium format before buying the D800. The price wasn't a major factor to me. However, the ISO performance was as I sometimes need to shoot indoors with ambient light only. I also considered the RED system as well since I am also a filmmaker, but the ISO performance wasn't there either.
There are times when I'd really like to do the shallow DOF of a MF camera, but for 95% of my uses, it's just not an issue.