I went from hg -> git in the last couple of years. I'd say they are "almost identical". The key difference is a slightly different mindset on merging and branching IMHO. It can be non-trivial for some of my more hard headed dev brethren to change their way of thinking once they've chosen a method and gotten in the correct mindset.
I went from using mercurial to git and I still have lots of respect for mercurial. I wouldn't mind the switch at all if I needed to work with a team who used mercurial, but personally I still just prefer git.
Yup, that's me in a nutshell. I used svk first and then moved on to mercurial.
Then I had to learn git. I like the model for lightweight branches in git (bookmarks in mercurial) but the step backwards in UI is still irritating. I'm used to it at this point but teaching it to others is a constant source of pain ("git checkout -t/-b"!)
Ugh, no, I fought my way through git first, and I didn't think hg was the same when I encountered it, I thought hg was nigh-unusable after I thrashed at it for a week.
I work for an agency that often picks up work from large agencies or in-house teams and most of the time I find myself using either Subversion or Mercurial. We use Mercurial for our own projects, but one client uses Git, which I use at home and prefer it to Mercurial.
It's a situation I imagine a lot of development agencies find themselves in.
The last company I worked at had a least a half dozen different dev teams working on more or less unrelated projects, all doing their own thing. While each team all used the same tools internally, the different teams used different tools and each team handled its own vcs hosting. I can imagine a company like that might want to get everybody on the same page, without forcing one team to give up their favorite tool.
I'm not aware of any, but I suppose this means you can use your preferred tools even if the rest of your team disagrees.
For my home projects, I used Mercurial for a year since I preferred its command-line tools to Git. When workable GUI tools for Git appeared (starting with Github for Windows), I migrated to Git.
Also, a small bravo to Fogcreek for using a screen grab from Windows. Windows is so uncool that we need some Windows screen shot affirmative action.