Just like JUnit makes people "test infected", darcs is getting me "version control infected." I'm using darcs for every little throwaway script I write now. Code a little, test a little, darcs record. It's dirt simple to use, and since there's no administration because there's no "central repository", you can create a new repository for any little thing you want to work on.
Darcs doesn't slow you down, and you'll be glad to have all the flexibility of being able to review (or revert to) older versions of your code. If you're doing something a little more complicated, that's even better. You can do branching, share patches with other people (maybe you want to work on something with a couple of other people, but can't or don't want to make a branch on the "official" project version control system). It's just cool.
You may want to use it even though you already have another system. Make little, fine-grained patches with darcs, and when you've got everything finalized, go ahead and check in your changes to the Official Repository using your Offically Approved Version Control System. It's nice to have a little more flexibility for your own workspace.