My team has been using Mercurial as our version control system, and it is definitely pretty cool.
We have run into some interesting “problems” though. Since the main Thunderbird repository is subversion, and we don’t have commit access, we have to submit patches that are reviewed and then committed. The real problem comes with branching, merging, and committing.
Long story made short (or the moral of the story) is that newbies shouldn’t push/pull Mercurial repos from each other. If they do, you end up with a mess that can only be solved by starting a new repo, and making your changes to that…
If you want more details, just post a comment and I’ll hook you up.