If someone writes a meaningful glowing review of the new Kindle, then there should be information and details in the content of the writing that explains why they think it is so great. If there is no such information in the article, then the article probably isn't very helpful, and neither the presence nor absence of affiliate links will change that.
But what if it is an awful fluffy review written for the express purpose of making an affiliate sale? Is this wrong, per se? If something in the article either directly or indirectly prompts a reader to click on the link and they make the purchase, then what harm has been done?
Newspapers likely disallow such practices in order to maintain journalistic integrity, but a blog author who is writing posts on purpose to sell things is probably not interested in maintaining journalistic integrity. The blog author is just interested in selling stuff. Maybe the blog posts are well-written and interesting, or maybe they are not. If they are not, then readers who care principally about content will likely avoid the blog on the lack of merit of the content itself.
A product review is close to useless if you have no trust in its author. The crux of a review is the author's opinion about something. Whatever verifiable facts are contained in the review are still only those facts the author chooses to highlight. Consider: if authorship doesn't matter, why is a review different from a press release? In short, a review cannot speak for itself because context matters.
Let's set aside the ambiguous case of adding affiliate links and talk about out-and-out payola. If a blogger takes a secret cash payment to write a false product review, then I believe that is dishonest and wrong no matter how well written or useful the review may be. (And not to confuse what's legal with what's ethical, but taking cash for reviews on your blog might even be illegal.)