I just got back from NAB and I can tell you that the Blackmagic 4K camera was the talk of the show.
What Blackmagic has done is listened to their customers who were greatly concerned with the 2.5K's rolling shutter and 16mm sensor, and addressed those issues quickly and without raising the price substantially. The camera image looks great (particularly when paired up with a nice Zeiss prime), and I do believe Blackmagic when they say they've learned their lesson and are serious about meeting their (late) July deadline. I think the biggest problem that they face is that they simply won't be able to keep up with demand, this puppy will be sold out for quite some time.
I do think this 4K camera will first and foremost damage RED Scarlet sales, followed by Canon C-line sales. I also think it spells doom for sales of the 5D3 or 7D2 to filmmakers. Canon and RED are in serious trouble and they know it.
There are some negatives to the Blackmagic 4K camera, first of all the crappy form factor (inputs on the wrong side, poor controls and lack of assignable buttons). Secondly, the 4K only shoots 4K, which means that if you also want to shoot HD or 2.5K with it, you simply can't (firmware update perhaps?). Oh, and there's that lack-of-swappable-battery issue that still causes puzzlement among the masses.
Having said that, for the price and image quality (not just about resolution, the image indeed looks stunning), I think most of us will be ok with the workarounds and lacks.
Moving onto the Blackmagic Mini. It's a very exciting camera, it lends itself well to a B-camera, and as a primary camera on shots not needing a super-low-light sensor or super-shallow DOF. For instance, establishing exterior shots, or greenscreen work. Rolling shutter on it is just as bad as its older brother, and the Super-16mm sized sensor will limit its usage for ultra-wide angle shots or CU needing very shallow DOF. However, this is a (potentially) quad-copter friendly camera which will also appeal to some. Other negative is needing to either purchase a whole slew of new lenses, or use EF to M4/3 adaptors...not ideal.
Do I wish that both cameras could do 60p or more? Of course, but as much as we tend to fixate on slow motion, how often does it get used on dramatic indie films which until now were mostly using the 5D?
So what can Canon (and RED) do now?
RED could drop the price of its Scarlet to match the BMCC 4K. Sure it would still be more expensive with the add-ons, but RED could argue quite well that the Scarlet can not only shoot 120fps, but is also a more production-ready and field-tested camera.
Canon should think long and hard about allowing its camera line (including the 5D3) to shoot 60fps at 1080p. They might consider updating the video codec to ProRes (could be possible). I sure hope that their uncompressed 422 HDMI out looks substantially better than what the camera currently records internally. Thankfully they can also rely on the multitude of still photographers who don't really give a crap about video.
Unfortunately their C line is...well...screwed. They could drop the price (substantially) which would undoubtedly piss off many recent customers. Even so, I really believe that for all intents and purposes, the C line is as good as dead since it will now live in a dreaded no-man's land between high end productions (ALEXA and Epic) and low end (Blackmagic).
From my very skeptical analysis (FYI, last year's BMCC 2.5k failed to impress), I think in this particular case the hype is well deserved. I think the BMCC 4K and Mini will (assuming they ship reasonably on schedule) have a huge impact on independent filmmaking.