I've converted to Mercurial and I had 2 main reasons:
1) not being able to compile monotone with gcc -- Mercurial is in Python
2) the need for custom 'server' and not being able to easily use one
machine for multiple projects (one running instance of monotone
handles only one database and I didn't want to put unreleated projects
to the same database) -- Mercurial can export repositories over http
using a Python cgi script (I can have as many different repositiories
exported as I want) and any user with the right to execute cgi script
can export repo without additional privileges