I've just been to a ClearCase training course by Island Training. It was really good: the instructor knew his stuff, made it not boring, and nicely catalyzed discussion about how it relates to our own build system. Tim says that HP's in-house training is always excellent and it seems to be true.
I have more respect for ClearCase now: in it's favour, it is a nicely generalized database, and it's very consistently unixy (mkview, lsview, rmview, ...). It avoids some problems with CVS, like not handling directory restructuring, and not being able to mark tags or branches obsolete without removing them. I even worked out how to do merges in emacs rather than through the silly built-in tools.
On the other hand, I think putting it into the kernel is a perfect example of a design that looks good at first but really ought to be dropped on consideration. (At the moment, our build machine is rebooting probably because ClearCase somehow interferes with loopback devices.) And it's still the perfect antidote for distcc's speed.