A while ago I wrote a entry called Subversion is a gateway drug. In general, my version control needs were fairly modest, and I tried to keep them that way. However, for some recent projects we have 2 to 3 people modifying code, and have the code in a few different versions. I had to do some branching to fold in changes to the stable version while doing development changes as well.
I found that, in general, even with cvs2svn doing the wrong thing with non-text files, it was easier to convert from CVS to SVN and branch, than to branch the CVS. I always ended up running problems with "branch is not a sticy tag" that I couldn't get resolved...
However, once I had branches, I started wanting to use them, and found that merging between branches was quite painful. I really have to have at least two branches, with all changes from one being merged into the other, and sometimes going the other direction when changes are ready. Really I want a tree of branches where changes at a higher level flow down to a lower level. That would be ideal.
I was looking at other systems, and finally with the help of Jeremy Hinegardner and Duncan McGreggor, I switched to darcs, and have been liking it.
Basically, with darcs, you can check out one working tree into another. Each of these is a self-sufficient repository. The repositories can be local or remote. So, I can have a parent repository and then children can "darcs pull" from their parents. I can also cherry-pick patches to push back up-stream. For me, that's a much less frequent thing, so that can be much more difficult. It's keeping the children up with the parents that happens frequently.
I like that I have a full local copy of the repository. It makes doing releases insanely easy and fast, plus I can do commits and tags without needing net access.
However, pushing and pulling from a remote repository can be real slow. Slower than svn at least.
I should try svk at some point. Really all I'd like to see is good branch merging support. I know you can do tricks to follow branches, but those seem like they require remembering much more and are more prone to errors.