After having used CVS and GNU arch (tla) before, we switched to Mercurial about a year ago for MoinMoin development.
As well as tla has been a big step forward compared to CVS, Mercurial is another big step forward from tla.
We don't regret it, Mercurial works great:
* we never had serious trouble with it, we never lost data
* it is FAST (much faster than GNU arch)
* the commands are easy to remember and have sane
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by herbwin
I have used it quite a while now and I am very happy with it. I also recommend using TortoiseHg which is also easy to use and has a very nice GUI front-end to Mercurial!
If you have never used a distributed version control tool before, this is the one!
http://tortoisehg.sourceforge.net/