ClearCase
My experience with ClearCase screams "PASS".
Come to think of it, all of Rational's products leave me feeling this way.
The key problem with ClearCase is that its too clever for its own good. Its implemented as a foreign file system. So it tries to pretend its part of your file system but the illusion isn't quite good enough. You have to use their file manipulation tools. So you can't say rm file, its ccrm file (or something like that). Its easy to forget whether the directory you are working on is a cc thing or a regular thing and pick the wrong tool.
It also entail rather tight OS integration and its upgrade cycle has held back critical OS patches from the dev group.
