When you receive a PR on GitHub it is boring to add a new remote for the submitter fork and to fetch his branch.
Fortunately, GH creates a remote ref for each PR, so you can use the following syntax to create a local branch that reflects the PR branch:
git fetch origin refs/pull/PR_NUMBER/head:LOCAL_BRANCH
Elegant and convenient!
git fetch origin refs/pull/611/head:pull_611
git checkout pull_611
/merge to get the ref to the pull request merged with its intended target branch.