Darcs manages collections of patches instead of chronological history. More about this is on the darcs wiki page documenting differences with Git. This difference is illustrated by a darcs ability to pull patches interactively, out of order. For example, you could pull all patches with a commit title that matched a ticket number from “development” to “testing”. Darcs would automatically make sure any other patches these patches depended on were pulled as well. With this key feature, you may need far fewer branches and repos to maintain. While in Git it’s very helpful to create a branch before doing work, in Darcs that’s often not a concern, as long as you your related commits mention the same ticket number. I used a workflow based on this extensively with a 100,000k LoC project. Besides that technical difference, darcs is very user-friendly. There are fewer commands, and most are interactive by default, prompting you about what to do.
Because of darcs’s strengths and ease of use, I much prefer it over git, which I also use regularly for open source projects. Darcs is easy enough to use that even if you have to learn git to contribute to some projects, you may still enjoy and benefit from using darcs on other projects where you have a choice.