When reviewing a teammate's code, I sometimes like to checkout their branch to review it in my editor or test to see if it fixes a bug. If I navigate to the page for the branch, I can click the … button to get more options, which include Check Out in SourceTree. It would be really handy if this was in the pull request Overview tab, too.

Note that I'm still on Bitbucket 4.9.1, so it's possible this has been added. I skimmed the release notes for newer versions and didn't catch it if it was there.