GitHub for Visual Studio 2.3 adds the ability to comment on pull requests directly from your IDE. Simply open a pull request in the GitHub pane in Visual Studio, and open a compare view, and you'll see existing comments right there in the editor. Click the icon in the margin to add new comments.
The functionality is limited right now to adding single comments, but we're hoping to bring the whole GitHub review experience right into Visual Studio soon!
On top of that, we've shipped a whole bunch of new features and bugfixes. To find out more check out the release notes.