I've used CVS, Clear case, SVN, and git before, and I think it's all over, and now we're using perforce.
The use of the same as used before, each user needs to establish a different workspace, and clear case of the view is similar to the need to enter some rules to set the required directory and branch. In addition now and clear quest combined with, should be considered a kind of UCM, each commit code, need to have a defect number, so that every version of each file can be found in CQ Source, all the changes are managed with CQ, It's a bit tougher to write a comment or no comment at all. But it's also a bit of a hassle.
P4 Although there are version trees, but used to the kind of clear case, see the other is a bit unaccustomed.
P4 oneself also has the comparison tool and the merger tool, but feels inferior to beyond compare to use, the merge tool not often uses, uses the Git time commonly, but always does not dare to use. P4 can easily modify and set up a third-party comparison tool, unlike the clear case that requires manual modification of the file.
Others are still familiar.