Hi; yes, I actually saw that on del.icio.us, and it's what got me thinking about version control again.
It sounds like the developer on Mercurial has the technical parts down well. I don't know about usability, especially since it seems like it's a contender for Linux development, and I think Linux developers (or maybe just Linus) have distorted ideas about usable version control.
I don't know what the technical foundation of Bazaar-NG is, but they seem the most concerned with basic usability issues. Other ones can be better or worse in terms of usability -- Arch is pretty horrible from what I can tell, and Darcs is pretty straight-forward. Many of them put so much emphasis on the distributed part, that continuous integrating (the norm in Subversion or CVS) is an Excersize Left Up To The Reader.
It is interesting that this latest generation of version control systems is very heavy on Python (Mercurial, Bazaar-NG, Codeville, and I think I'm forgetting something else...)
From a usability perspective, Mercurial is quite similar to both CVS and SVN. Commands that "mean the same" in CVS and Mercurial have the same names, flags, and so on.
Linus definitely has extremely peculiar notions about version control, but those are all firewalled in git/cogito, and have not bled elsewhere.