Surprised no one's mentioned "waterfall", the stuff is magic because it moves.
I've had no problems with cell. breaking down, though I've seen plenty of it. In my opinion, it's the really cheap "10 cent", give away promotional, and advertising knives that suffer most. High quality celluloid is expensive and time consuming to make. It must be properly cured for months before being used. If cured properly, it's very stable. The oldest celluloid knives I have are "French Ivory" examples from Sheffield and most of them are over 100 yrs old and show no signs of deteriorating. I think the more volatile cell. came about after the turn of the century when mass production and cost cutting became more important than craftsmanship and quality. If you stick with the early brands from Sheffield and New England I wouldn't worry too much. Of course, the new stuff is imitation cell. and shouldn't be a problem either.
Celluloid greatest virture is that it can be made to look like just about anything. I've seen plenty of butt ugly pieces that looked like failed experiments for every one of the breathtaking pieces you guys have posted pixs of. That's one of the reasons I like this hobby, there's something for everyone.