I like SourceTree but it the UI gets clunky and slow within a short period of time. Not sure why, but it usually happens when you have anything more than a trivial number of commits.

Have you tried it in the last few days? After SourceTree got relatively slow for me with the past few updates, the last update actually completely fixed that!

I use SourceTree for git, but I find TortoiseHg is better for mercurial; it's just far more feature complete, with support for almost all mercurial commands and (bundled) extensions.

I'm still somewhat surprised by how poor git gui's are, considering it's the more popular VCS - though SourceTree is certainly one of the better ones.

I was really happy when I heard about SourceTree(back then it was not even released yet) but when it finally came out it was bulky to begin with.

TortoiseHg on the other hand always looked nice and clean.