I was a fan of Darcs for years. It's easy to use, very easy to understand, and it served me well. The occasional merge could be... problematic, but eventually I had to change because my main project got too big for Darcs.
The command I ran most often 'darcs whatsnew' took 30-40 seconds to run. Importing the repository into git speed things up a little: 'git status' now takes 0.4 seconds, about 100 times as fast! I still have a soft spot in my heart for Darcs, but all my projects are now managed by git.