The reason the darcs merging algorithm is talked about is because the algorithm has a huge impact on how the program works. The big problem is that if you try to merge two substantially similar patches, you'll get a conflict which kicks off a comparison against every line of every patch you've ever put into the system. Needless to say, this causes the app to hang (sometimes for hours) and is thus a subject for debate. You rarely run into this, but when you do it's annoying to have to do the dance to get your repo in working order again.
darcs is my favorite vcs because it's terrifically simple and by default has interactive prompting for patches and you can push or pull between repos. The email stuff is just another transport, normal transport is via scp (transparent when using push/pull). I wish it didn't have the merge problem so I could recommend it to people.
The only two dvcs systems I'd look at today are svk and bzr.
Bah, re-read the darcs conflict bug when I posted and realized I didn't describe the conflict correctly.
The conflict compares every line of every patch to every line of every other patch that you've ever committed to the system. It's very ugly when it hits.