I've shot two films on the 5D (check my IMDB page) and our third will likely be shot on the D800 if it's up to the task. But the support system needed to shoot DSLR properly for that kind of use often outstrips the price of the camera. Unlike a camcorder, DSLRs don't do video well if you simply hold them in your hands.
Editing DSLR video is no picnic either. All the people here complaining about how big the NEFs are from the D800... Start playing with the video and see what happens to your hard drive space. There's a reason I have 10TB available for my film projects.
For anyone who is NOT trying to make a film, a camcorder is probably the right answer 99% of the time. Shooting video in low light is a bad proposition no matter WHAT gear you have. VERY few people serious about it will push beyond 800 ISO.