Some years ago, when I start working with mercurial, I looked at a nice GUI. Of course a GUI isn't necessary to use hg. But I think this is a good thing, even if you know how to use hg in cli.
Aproximatively at the same time, I bought a MacBook. And you know what? With a mac you can have SourceTree! SourceTree was acquired by Atlassian at the end of 2011.
SourceTree is just an awesome GUI for hg and git! Why? Because it's not a basic client. You can just do all you want with source tree. You can manage hg and git with the same client. Browsing history, branching, stagging, merging, etc. All works fine and fluently.
And if you use DCVS in such a "classical" way (with develop branch, deployable/release branches) you can use, directly from the interface, gitflow from Vincent Driessen (and now hgflow too). I believe that no other integrates this. And that's realy a good point in favor of SourceTree.
Every time I start my laptop at work and boot... windows... F * * ck, no SourceTree here... I tried many other git and mercurial GUI, but after using SourceTree, all are so... hum... No one is at the level of SourceTree.
The good point is that force you to learn git and hg in command line. Ok, even with a nice gui you must learn using git and hg in command line.
Later, github released two git gui, one for mac and one for windows. The gui are very simple, but do the job. Not more. That's a good choice, if you only use git... (honnestly the mac github client is pretty nice).
The 22 of february, I read a good news: SourceTree available in beta for window! :-)
Just after reading that, I download and install it! Finally a good hg gui for windows! And git too!
Today, SourceTree is officially available for windows. And it's free!
Hey, Atlassian, do you want to be really awesome? Make it available for linux ;-)
