There are really three situations you face when reading a negative review: you either agree with it, don't agree with it or don't know. For me, with each situation I'll usually comment.
If I agree with the review, I generally state why I agree with the reviewer.
If I haven't read the book, I generally comment and say it was a bummer the reader didn't like it but if it's something I want to read, I'll still read it to decide myself.
If I don't agree with the review, it depends on my mood. If I really love the book and someone doesn't like it, I'll usually comment on the review and respectfully disagree. But this all depends on the review itself.
There are two types of negative reviews. There is the respectful review that looks at the book and can constructively and politely say why they didn't like it. Then there is the not-so-respectful reviews that trash on a book.
If it's a respectful review, I'll usually comment and respectfully disagree with reviewer. But if it's a disrespectful reviewer, I will not comment. No matter how mad the review makes me I don't comment. I might yell at the screen, call the person and idiot and thrash around the house for a couple of minutes gaining the satisfaction of scaring my cats but I never comment. Generally speaking, reviews that bash the author or the book are just meant to draw attention and aren't worth my time.
I think the key to commenting on a negative review is the same as writing a negative review. You must be respectful because you really never know who is looking.
So what about you? Do you comment on negative reviews or steer clear of them like the plague?