I have only done a couple of reviews, and although I wasn’t vastly impressed with one of the sites I thought I gave a fair review, and mentioned what I liked and what I didn’t. And as you say, the part about the review being sponsored was clearly visible, but not in poor taste (such as having ’sponsored post’ in the URL).

And I think at the very least, even if you hate the product and give a bad review you can give them a proper link that isn’t nofollowed to say thanks for the $$$.

I think people need to give the advertiser a little more credit.