I'm blown away by the technology. The images at these low light levels are limited more by the quality of the light than the quantity.
Everyone talks about the tracking capability of the IV, but it is also really nice to have a high resolution LCD to chimp the images. When I shoot basketball, it is common to be able to see the veins of bloodshot eyes in the LCD. Needless to say, it is easy to tell if I have the skin tones right when the chimped image is that good.
Thumbs up on the 1D Mark IV.