I have had some unpleasant issues with darcs working on cl-json, and I find that working with it is very badly hampered by its lack of mindshare compared with other DVCSes. In particular:
1. The darcs maintainers are unable to supply binaries for Mac OSX. 2. No one has stepped up to the plate to provide a graphical UI for working with darcs.
Not only are these a nuisance in and of themselves (the former especially, since darcs does not build on an out-of-the-box install of the Haskell Platform), they are the signs of a tool that does not have an adequately robust support community.
Combine this with the oddity of the usage model, compared with other DCVSes,[1] and you have real problems.
I am not volunteering to maintain CL-JSON, so it is ultimately not up to me. But I have submitted a number of patches, so I feel that I have standing at least to offer the suggestion.
Note that I don't think that darcs, in and of itself, is a bad system. But there are more than enough DCVSes out there, and we can use one less of them.
Best regards, Robert
[1] See, for example: http://darcs.net/FAQ#how-do-i-go-back-to-an-older-version-of-my-repository