The Canon 5D Mark III has become a true cinema camera. That's a bold statement, but some of the footage that is currently being shot with the new 14-bit RAW Magic Lantern hack is absolutely night and day when compared to 8-bit H.264 footage recorded in-camera, and there is no doubt it rivals much more expensive cameras just in sheer image quality. We've got some impressive samples from cinema5D, Luke Neumann, as well as a few others below. Click through for the head-exploding videos.
Here is an impressive comparison by Saad Rabia, taking a look at the hacked Canon 5D Mark III, unhacked Mark III, and the GH3 (night footage shot at ISO 1,250):
Here is a larger frame grab of the night shot (click for larger, you can also download both videos from Vimeo to see the full 1080):
Here is a sample from cinema5D (you can also find DNGs on their site):
There are a few bugs to work out at the moment, especially since this is so early in the life of the hack. Right now there is no audio in-camera and clips are limited to 4GB (which is about a minute of footage), but the focus has been on actually getting it working and stable before moving on to the other issues. The 4GB limit is not actually even a hardware limitation, it's just the way the code is at the moment, so we will definitely see longer clip lengths. For the best quality, 1000x speed cards are recommended, like the Lexar 1000x cards (some people have also recommended the Komputarbay 1000x cards, but I have no history with the company, so you'll have to take someone else's word for it).
Many have been asking about different cameras. Unless you're a tester, you've just got to wait it out and see what will happen. I know it is working on cameras like the 5D Mark II and T3i, but none of them are currently able to do the full 1920 x 1080 (and they may never be able to), with the T3i coming in much lower resolution. Card speed is really the issue for any of the cameras that are SD card only, and it may never be possible to get results anywhere near what the Mark III is currently getting.
If you want to try it yourself, it's definitely not an easy process getting ML onto your camera, so I would suggest most people wait until Magic Lantern releases a complete build, which I imagine will happen in a matter of weeks. Magic Lantern has a forum dedicated to workflow, and cinema5D also put together their own guide for those adventurous folks who would like to try it out.
Stay tuned as we will have much more on this, including what this might mean for you and other camera purchases.