I work in a configuration management section for a large government IT contract in the UK. We use PVCS Dimensions and Clearcase, and unfortunately its my job to administrate both systems.
They both have very good points... they also can be a nightmare!
Clearcase is a pain to administer, and when it goes wrong (which is quite allot with the amount of developers using it in our place) it can be hard to work out what’s wrong.. and even harder to fix it! Also there is very little security in ClearCase.. unless you buy bolt ons from other companies.
Dimensions is a good and reliable tool.. but its not as flexible as clearcase. The good thing is that they both keep me in a job :o)