For what it's worth, I was in the Monotone [1] camp against Darcs back in the day... :)
I bet that if any version control system is going to replace git, it'll look at least as much like Darcs as anything else.
I'm no expert by a long shot, but to me it appears that if we're going to try to finally make version control user-friendly, something inspired by Darcs' patch algebra may well be at the base of it. While Darcs is impractical for a whole set of use cases, it's very practical for a whole other set of use cases.
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.
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.
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.
People said this about Subversion not so long ago.
I think it is good to periodically evaluate the tools used. And initially there was quite a bit of excitement around DVCS and everyone was playing with them (including me). Me and most have settled on one tool (git). And actually this was not my preferred tool of choice, I like Bazaar, but 2 things happened: 1) Bazaar has not won the popular vote (so chances are on a new team I would have to learn git anyway) and 2) DVCS as a technology has stopped being new and exciting.
For example at some point OO features and garbage collection was exciting and new. Now they are just another tool in the shed available for use and as I said, this area of tech for me has lost the "wow" factor.
I also realized I have a limited bandwidth both to learn and to remember things. I would rather devote this bandwidth to learn new languages or other technologies (databases) rather then learn more DVC systems.
Their way of doing things seems to require a 1000 times less code than current mainstream techniques. They can do a whole language stack in less than 2000 lines of self-implementing code for instance (not super-optimized, but fast enough). Compare with GCC or GHC. Even Lua is bigger.
Speaking of terseness, reminds me of Prolog. I remember thinking how beautiful the solutions are but how broken my brain is as I couldn't come up with them on my own. I could see the final result that someone else produced and understand it, but struggle for hours and hours myself.
I think at some point levels of meta and abstractions can go that high that only very few smart individuals can effectively create products, others more or less hit a wall.
When the teacher is a genius, the student needs not be bright.
For a long time, darcs didn't have much of a developer community, possibly due to the choice of implementation language (Haskell). It also had a corner case that could cause a merge to take several days to complete (I suppose they solved it by now; it was rare, but if you hit it, you couldn't really keep using darcs).
Maybe for open source projects it makes sense to stick with the most popular tool but for private teams if you can see an advantage to another tool and it looks like it will be around for a while then there is no reason not to use it.
I like Bazaar and used Hg, but this is tool used to collaborate with others (unlike say an editor) so picking a tool that others know and use is important in a team. Darcs has an interesting approach, sure, but that is not enough for it to win mind-share, and I wouldn't spend time looking at bit because I will have a very hard time getting others to do the same.
Also the excitement and euphoria about learning and experimenting with distributed VCs is passed. I feel, for most they have stopped having the "wow" factor and became just a tool in the same category as patch, diff, less and tar.