It is fine to write reviews about the products you promote. It is encouraged even. That being said, make sure that your review is honest. You need to genuinely represent the product you are promoting, which means pointing out its flaws as well as its qualities. If you focus only on the good to the exclusion of the bad, then people will distrust what you have to say and abandon your website. Affiliate marketing thrives on credibility.
But I think the biggest deciding factor in this, goes back to the site as a whole and all of the other posts. Are the genuine? Is the blogger constantly trying to push products? I’d like to think I’ve been doing this long enough that my audience knows I’m not out to make a quick buck – and I think even relatively new bloggers can prove this based on their other content.