So, what's the thoughts here about the $1995.00 Black Magic cinema camera, with EF mount? Pros? Cons? How well would it fit into my equipment bag for video, especially in light of the 5D3 having full RAW in the predictable future?
I have the BMCC and I sometimes use my wife's 5D3 for video (less now that I have the BMCC). Here are some thoughts on your questions and my experiences with the BMCC as someone who loved shooting video on my (and my wife's) Canon DSLRs.
First, I would say that the BMCC would not make a good 2nd camera. That's because it makes a good 1st camera! The video from the BMCC is better than the 5D3 in almost every way when comparing the BMCC's compressed (ProRes or DNxHD modes) or raw to the 5D3 compressed (H.264). The picture quality is really so much better from the BMCC. It does suffer aliasing/moire issues, but not as badly as the 5D Mark II or other Canon DSLRs. It's resolution and color are just so much nicer than the 5D3, in my opinion, that it makes up for the aliasing/moire.
When comparing the BMCC to the 5D3's raw, the BMCC still has an advantage, though it is narrowed. The advantages of the 5D3 now become: lack of aliasing/moire, better low light performance, and the full frame aesthetic. The negatives to the 5D3 (besides having to deal with ML) are: short recording time on available media and expensive media (compared to high capacity and relatively inexpensive SSD drives for the BMCC).
Also, the cropped sensor of the BMCC takes some getting used to when coming from the 5D3's full frame. Depending on what lenses you have, this might be an issue for you. My favorite lenses on the 5D3 (my 50mm and 85mm) rarely get used on the BMCC, since my favorite focal lengths on the BMCC are 35mm or less. Plus, the shallow depth of field is possible on the BMCC, but it isn't just there like it is on the 5D3 - you have to work for it.
Shooting with the BMCC is different than shooting with 5D3, for most of the above reasons, but for others, too. There is no histogram, the white balance choices are limited, there are no audio meters,... But, most of these issues are no big deal, once you learn the camera a bit. Plus, the footage (raw or even ProRes/DNxHD) is so flexible, that you can overexpose just a little or have the white balance really wrong and still get a good image in post.
Depending on how you shoot, the form factor is also an issue. The BMCC is, in my view, clunky. It is a heavy box with a long protruding lens. It isn't pretty and it is awkward to hand-hold. If you always shoot on a tripod, then it is no big deal. If you have a shoulder rig, then it is just a bit heavier. If you shoot hand-held, .... well,... switch to a tripod or a rig. With a lens with IS, you can shoot handheld and I do. But, it just isn't like the 5D3.
Then there is the battery. Internal only, not replaceable. Lasts about 45 minutes. An external battery is a must. There are some decent ones and they're not too expensive. But, they add to the bulk of the camera and equipment.
The form factor was also an odd fit in my camera bag. I had to rearrange my bag in a way that I didn't like. It just didn't fit well in my DSLR backpack. I finally ended up getting a Pelican case that holds my BMCC and rig. I now have a separate Pelican case for lenses. So, I feel less nimble with the BMCC than I did with the DSLR or even my huge EX1.
But, for all of these negatives, I just love the image I get out of the BMCC. I bought it for the raw capability, but the compressed mode (using the log FILM profile) is so great that I don't shoot raw much. Don't get me wrong - raw is great and it is better if I know there will be moire, some moire can be almost eliminated from the raw files. In compressed mode, I can shoot for hours on an SSD. Then, I just plug the SSD into the computer and edit from there. You don't get the drop in quality in going from raw to compressed on the BMCC the way you do on the 5D3. In fact, on a lot of BMCC material, you probably couldn't tell if it was shot in raw vs. compressed.
In summary, for video, the BMCC might become your #1 camera. The crop factor might have you preferring different lenses than you do with your 5D3. The form factor is definitely a negative coming from the 5D3. But, if an awesome image is your highest priority, the BMCC is hard to beat at that price.
I thought about a Pocket Camera as a second cam. For as little as I would use a second cam, I decided to use the 5D3 when I needed one. Problem is, the raw on the 5D3 is such a pain (mainly the short time on a single CF card) that I rarely use it. The even more extreme crop on the Pocket Camera (and need for new lenses) are definitely negatives to that camera for me. I might even pick up a second BMCC EF! At this price, it is tempting.