I know I sound like Mr Negative tonight, but I'm afraid that is just completely, absolutely and utterly wrong.
edit: Sorry, I should be more constructive.
It's simply that a) something being just an opinion doesn't get one off the hook at all and b) nor does it matter if it's "just one person's" personal opinion.
Yes, there's "fair comment" but it doesn't actually give much wriggle room.
I've got a lot of experience of [specifically British, mainly] libel law and, believe me, there's an awful amount here that is certainly libellous.
As I say, though, I can't imagine a suit coming from it (for lots of different reasons).
So I think that effectively it's only, really, moral considerations that guide us here as to what we feel we should or should not say.
But, likewise, JMO and sorry for curtness.