This is great, particularly the "Update Branch" feature. Prior to that, the only way to update a branch (or a fork for that matter) was to use an external client to merge in upstream changes and push back to GitHub.

I worked on a project with several non-developer users updating content. The web interface works well for simple HTML/CSS changes, but if we developers moved or renamed a file, their forks would immediately be useless. This feature should make that a non-issue.