Anna, I've also had people where their comment seemed to be b/c they were against my review.  Usually it is that I liked the product and they write that the product sucks.  At times it is as if they think I am the product creator and they are sending their negative feedback to me as if I can do anything about it :-D

Usually I respond with Sorry you didn't like but I did, maybe you should review it yourself too.

Sometimes I think people don't get that we are not writing product endorsements - we are writing our opinion.  Telling me all the things you hate about the product isn't going to change my opinion of it (unless you tell me it's made of posion!)