I used Clearcase (on Solaris) in 1999 and was not a fan. It slowed our build times by at least 10x. I'm sure it was probably set up wrong, but this was a Fortune 100 company with lots of dedicated resources.
Clearcase performance for many builds was specifically impacted by the very poor performance of stat(). You could make very real improvements on build times by reducing the number of calls to stat(). It was sort of amazing.
Clearcase also suffered, at least in my experience, from a clumsy and ugly merging process and deeply unintuitive command set which meant everyone who "used clearcase" actually tended to use some terrible homegrown wrapper scripts.
Still, considering it was the last remaining vestige of the Apollo Domain OS, not bad.
I used Clearcase a while back in an office in China. Every few days there'd be an email going round pleading with people to keep their antivirus software up-to-date; because someone had a virus somewhere, and apparently the virus would try to infect executables on the Clearcase virtual drive, at which point Clearcase would obligingly check the infected file into the VCS and distribute it to all clients...
Clearcase also suffered, at least in my experience, from a clumsy and ugly merging process and deeply unintuitive command set which meant everyone who "used clearcase" actually tended to use some terrible homegrown wrapper scripts.
Still, considering it was the last remaining vestige of the Apollo Domain OS, not bad.