So as to the steep learning curve --- it took me a day or two to being able to do what I want, so it seems to me it's really easy to learn once you know some distributed vcs. I found out the documentation is much better than in mercurial --- in git one just types "git help command" and a full man page fires up with lot's of information and examples. In mercurial, one just gets a couple lines (compared to git) of help in the terminal, without examples. Also the first (actually second) thing I noticed is that "git log" (and other commands like "git diff") doesn't scroll out my terminal, but uses "less" automatically -- that's just great. The first thing I noticed is that git is superfast for almost every operation, especially it's noticeable with "git diff". As to the community, I only had a chance to send a few emails to the git list, so I cannot really say, but so far it's very responsive and friendly.
However, those are just little things that can (and I am sure they will) be fixed in mercurial as well. What is a big thing are branches, especially remote branches. One just fetches a remote repository and then works with it like with any other local branch, e.g. one can switch to it, or just "git log some_remote_branch" to see what is in there. One can easily compare them etc. With mercurial, I was using "hg out" and "hg in" commands to see what changes I will pull or push, but those commands require internet connection, so it really sucks and it's slow. In mercurial, I was using different directories for different branches, but that's just extremely inconvenient and also slow (creating a new branch is just a matter of adding one file, not copying the whole repository --- for example with the sympy hg repository, I often had to wait several seconds to clone a repo, while with the git repository it's just instant) and big (in terms of megabytes).
The bible of Mercurial, the hgbook says:
In most instances, isolating branches in repositories is the right approach.
But I think it's the wrong approach. The only argument for doing this that I accept is that sometimes you are not sure which branch you are in in git --- well, I use this trick (actually, just read the documentation of __git_ps1 in /etc/bash_completion.d/git), so my bash line always says which branch I am in using the red color. I never do any mistakes with this.
Anyways, I could continue like this (e.g. check out "git svn", "git rebase -i" and many other things), but you can read why git is better for example here, no need to repeat here. I am very happy now and I can only recommend to learn git.
So let me also say some things that are worse in git --- one thing that could be improved are URLs of the gitweb (hgweb urls are neat, gitweb urls use "?id=..." stuff). Another thing is that the debian package git is not git! You need to install git-core to get git (but this seems to be getting fixed).
There is also an interesting discussion happening right now in the debian-python mailinglist about switching the DPMT repositories to git. I think almost all opinions (for and against) were already stated, but if you have some opinion that wasn't yet said, please do so.