The side-by-side diff viewer works great.  The text lines up very well, and is very easy to use.

My only complaint is the section between the diffs that tries to help you see exactly where the new or changed sections are within the old.  At the top, it is fine, but as you scroll down, the transitions move up slightly faster that the code they represent.  In large files, this becomes especially notable.  Here is a narrow screenshot.

When you look closely, you can see which parts of the code the connectors are trying to line up, but you have to mentally make that shift downward to make sense of it.