I try to review edits as good as I can. Mostly these are corrections in spelling or formatting changes, which is fine. But now I just encountered an edit where another user added completely new content to an existing answer. 

I'm not able to judge if this new content is really useful or accurate (it could even result in downvotes of the answer if the new content is bogus). How should edits that add new content to an answer be handled? Should I just approve it and let the OP rollback if he desires? How do the mods handle edits like this?