I agree, SourceTree is very nice. Personally, I prefer the command line (with bash bindings like gc for git commit) but I use SourceTree for visualisation which I find much nicer than the one git, github or bitbucket (also Atlassian) offer.
To bad it only supports OS X. Many of my co-developers use Windows or Linux and would definitely like this tool.
I'm in the same boat, love the command line for branch/merge, but day to day using Ubuntu/Windows XP - so I find SmartGIT the best choice here - really nice for visualisation of the commit log/diffs/etc.