In my opinion writing reviews for affiliate items all depends on whether or not you are promoting those items on your blog or website.
The problem that I have with most positive reviews is that the people writing the reviews are normally associated in some way with the product. In my opinion that takes away most of the credibility of the review. It is very difficult for many people to stay unbiased in their reviews if they have a monetary gain in the product selling. If you are promoting the product on your blog or website how can you honestly give it an unbiased review.
Doing studies on the review process and a lot of research on who is actually doing the reviews, many of the reviews posted on the web are written by professional review organizations and individuals that gain from the product itself. So with that said, most of us put very little credence in the actual review knowing that is one way or another it is going to be slanted based on how the writer wants the review perceived.
So if you really believe in the product, go for it and see if it is a way to generate any type of revenue. But if you are doing it merely to generate revenue with out truly believing in the product or even using the product, I can not do that. My integrity is worth more that a couple of bucks for misleading an unsuspecting audience.
Just my thoughts.....