my take on where RCS is now
Back when OSAF was looking into replacing CVS about the only stable and wide-spread option I found was Subversion and while it had some flaws, compared to CVS it was much better. At the time I had heard of other projects using things like BitKeeper and Perforce and other commercial apps., but that's the kicker - they were commercial and as an Open Source shop we really wanted to use Open Source tools.
So we went with Subversion and really haven't had any regrets. Merges and branches were much saner and after the Subversion team released the FSFS (filesystem storage) the server side repository issues went away.
Recently tho people have been asking me about distributed version control systems like mercurial, bazaar and darcs.
The first two I've been following indirectly by keeping track of the work the Mozilla build/release team have done in trying to pick their next system. See J. Paul Reeds blog entry Version Control System Redux Redux and also their wiki page for details. The short-form result is they haven't chosen one yet.
darcs I've been following more directly because I've started using it to try and help the Buildbot team with some patch and bug work. So far, while it's been different, I haven't had any issues but I also haven't gotten much beyond the more basic operations. I plan on posting a more detailed look at it after I do some true distributed operations.
One of the newer programs (for me that is) that has hit the nets with some force is GIT but I think that is because recently Linus Torvalds recently gave a talk at Google about it and seems to be on a tear promoting it :)
But it really seems to be worth looking into. A recent article about it's design, GIT for Computer Scientists, really showed some interesting internal concepts and I like what I see. So I need to figure out how to work it into my project space to give it a fair look.
There are a couple other distributed systems I haven't talked about, Monotone and Arch, but that's because I know next to nothing about them. If you want to find out details on them I would recommend reading the Version Control Blog for articles about them - it's a great read.