I run a blog which I would estimate is about 90% high quality free content and the rest affiliate reviews. One thing that bugs me is when affiliate marketers just copy and paste bits and pieces of content from the sales page and maybe add a short sentence or two to lead into their own affiliate link. Affiliate marketing gets a bad name from this practice.
A good product recommendation should be real review. The product should be purchased, tested and then written about honestly, describing pluses and minuses, in the form of a thorough report. Using screen shots of your own findings also helps. Video is also useful in reviews, but not a sales video from the sales page!