In case you are not aware (it isn't clear from your post), darcs is among the oldest of the DVCS, making an appearance in 2003 - it predates git, hg, bzr (bazaar v2) and fossil. It came along after arch and baz (bazaar v1), and more or less together with monotone (which was a source of inspiration for git).

For a long time, darcs didn't have much of a developer community, possibly due to the choice of implementation language (Haskell). It also had a corner case that could cause a merge to take several days to complete (I suppose they solved it by now; it was rare, but if you hit it, you couldn't really keep using darcs).