It will be very hard for Darcs (and Bazaar, Monotone, etc.) to get so much better than git that switching is worth it. Just a better theoretical base for some edge cases is not enough.
Hg has also quite some mindshare. I consider it the only DVCS, which still seriously competes with Git. Apart from that, there are niche groups: Haskell with Darcs, Ubuntu with Bazaar.
Fwiw he didn't say Darcs will replace git, he said that whatever eventually replaces git will likely implement some of Darcs best features like the patch algebra.
Hence, it's worth taking a little time to actually understand those features, even if you never use darcs. Plus the mental exercise and learning something new is valuable in and of itself.
> Just a better theoretical base for some edge cases is not enough.
That's why I said "user friendliness". Even on a superficial level, Git is enormously lacking in this aspect. More fundamentally, the way moving changes around between branches works in Darcs is significantly simpler and "hey, it read my mind!" than in Git, where you need to understand the underlying object model and the tools very well to do this well. Sure, many other things are a disaster in Darcs, but I'm not saying Darcs will replace Git. I'm saying something inspired by it well.
Hg has also quite some mindshare. I consider it the only DVCS, which still seriously competes with Git. Apart from that, there are niche groups: Haskell with Darcs, Ubuntu with Bazaar.