edit: My thoughts after using it.
- vim-github-commit doesn't use inline comments, the comment goes at the end of the file on GitHub, although it does say which line it refers to. I prefer GitHub's inline comments via the web interface.
- Your comment is added to whatever commit the current line was introduced in. This isn't bad, but it can be confusing if you go to GitHub and realize you wanted to comment on that line in the current version, not some version from 6 months ago.
- The comment cannot be edited or deleted (as far as I can tell), whereas you can do both if you make a comment via the web interface.
Here's how it compares to a regular comment on GitHub: http://i.imgur.com/QsDNOSu.png