Your question is difficult to answer in the light in which you intended it, because it doesn't really apply to me. I am not so invested in my source control system to say it "rocks". It works for my purposes and maps nicely to my view of how to handle source code--it's less that Mercurial is awesome, it's that I find Git unpleasant to work with.
I find Git over-engineered toward the wrong goals; while I don't doubt the development credentials of the Git developers, I think that too little time has been put into making developers' lives easier. Linus has said previously that speed was a priority for him, and that's great--for kernel sources, I'd definitely put a premium on speed. It appears, however, to come at a cost: Git has long struck me as extremely user-hostile. The documentation may have improved as of late (I haven't checked) but it was absolutely atrocious for a very long time. I find its interface unintuitive and nondiscoverable (and here hg-git could help, were it not for the cognitive dissonance of dealing with one against the other). And its Windows support, even today, is unacceptably poor given that I spend about 40% of my time working there.
hg-git addresses some of the more cosmetic issues noted above that I could, if so inclined, deal with myself, but it doesn't really address deeper concerns I have with Git itself. There are also issues of philosophy involved--Git requires you to accept certain behaviors and the cultural encouragement of same, e.g. rewritable history, of which I'm very much not a fan. (You can rewrite a Mercurial history as well, but it requires going out of your way to do so and is not culturally accepted. "Rebase" exists, via extensions, but nobody uses it. And I much prefer it that way.)
To me, Mercurial feels much more like its developers spent a bit more time thinking--looking at how a wider set of developers work, and building something that's comfortable to more people in less time. Git feels like its developers asked no questions at all and started hammering out what made sense to them. Which is fine when it maps to the way you think and the way you work, but it doesn't map to either for me.
(I also tend to think that Git wouldn't be nearly as popular had it not been reflexively adopted by everyone and their brother because of Github--this isn't "waah, my tool of choice isn't popular!", but more of a simple observation. I think it succeeds more in spite of itself than anything.)