My main problem with it at first glance is that in the month view it does not highlight which days have events, so you need to click to see if there’s anything on that day.
I am not the biggest fan of SourceTree's UI, but the killer feature that makes it really useful for me is that it makes it pretty easy to stage specific lines of a file or to discard some lines. They've done a pretty good job with that and I use those features all the time now.