For my personal projects, I've used Mercurial Hg for a while, but I've since been migrating to Git.
Hg seems pretty responsive for the most part, and I haven't seen it corrupt any data. One time I wasn't able to pull changes from another repository, due to a bug or version difference inside Hg, so that was a bit of a turn off.
I haven't looked very hard, but I couldn't immediately find any good gui tools for it. I've chosen to abandon it in favor of Git.
There seems to be a bit of a war going on being what's better. Personally I don't like the fact that Hg is written in Python. I'll change my mind about Python once I see the dependency and version problems go away. I think it might have gotten a lot better though. But I still don't like the way Python source code looks.
One thing I do like about it is that it seems to let you concentrate business logic close together.
Python isn't the only reason I abandoned Hg. Check out my Git page for more.