I just use Genre...
I use the following:
TV Shows
Animation
Movies
Comedy
Music Video
Misc
(and then I have a bunch of "video editing" and "work" related categories that aren't directly related to your issue)
You could just as easily use Category as someone else suggested. I considered doing this, as I may someday actually categorize my Movies and other video into true "Genres" (like Suspense, Horror, Comedy, etc), but I just use Genre for now. If I ever do bother to sub-categorize, the Move/Copy fields tool is good enough to move the data over to a "real" category field I suppose...
Of course, I do have a 1 TB RAID5 volume that is used for all my "library" video (as opposed to the 350 GB BeyondTV "DVR" drive), so my categorizations are purely for MC organizational purposes. They still do come in handy when I'm offloading things I've watched (and aren't likely to do again anytime soon) onto DVD-Rs. You wouldn't really want to burn off some random movie with an episode of CSI or Mork and Mindy... If you're serious about storing video though, I highly recommend you reconsider the RAID. It doesn't eliminate all risk, but it certainly mitigates some of it. And it just makes working with, and storing, the video files so much simpler and "cleaner".