Announced today, GitHub for Windows is the most beautiful modern DVCS client UI currently available. It has a well-thought-out Metro look-and-feel. I tried Git via GitHub back in 2009 only to switch to – at the time – more straightforward Mercurial tools (I use TortoiseHg and VisualHg for Visual Studio Integration).
I don't think this alone would cause me to switch from Mercurial but at the very least it is a bold step forward in DVCS client user experience that others can learn from.
Some basic screen shots below. More screenshots at the original announcement.