I've been using mercurial for awhile now, for conary (hg.rpath.com), but in doing so I've only really used basic functionality. I've really liked that the distributed model allows me to work when not connected to a network, as is necessary for systems such as CVS. Anyway, an issue came up where it would be desirable to use source control to work on a third party project, namely fedora/redhat initscripts, which are in a CVS repo. I went to the mercurial website to see if there were conversion tools, and noticed a project (done by the guys who write darcs, yet another source control system) to push and pull from one system to another. I set up a simple config file (mostly machine generated) and pulled the entire archive from anon cvs into an hg repo. I then set up a cron job to pull from fedora hourly and dump into that repo and now I can merge from fedora to mine whenever necessary. To put it mildly, this is better than the mass o patches I was otherwise faced with. Mercurial makes this unbelievably easy, and if you haven't checked out mercurial and tailor, you should do so now :-).