I have been contemplating the migration from Subversion to Mercurial for a long while now.
I'll probably try the migration sometime soon.
If you are using the source code and have a problem with this, please discuss in this thread.
The advantages of Mercurial over SVN will be easier merge of external contributions, easier merges of branches, branches history more accessible, history replication everywhere as backup.
The CodingTeam forge where the code lives already supports Mercurial.