They aren't going to replace a consumer handycam.
The fast aperture primes really require you to be on the ball, or you'll end up with a lot of soft footage, and as discussed, the 50 f1.8 isn't mechanically all that friendly for MF in video.
I have a sigma 18-50 f28 DC Macro, which was reasonably cheap, bright in low light, and nice enough to use, nice wide zoom and focus rings, nice amount of throw.
I would think zone focusing... Keep the lens wide, aperture small, hyperfocus and you shouldn't need to refocus that much for most run and gun type stuff.
DSLRs are great for video if you are prepared to do the work, they are not really replacements for automated family camcorders.