I personally prefer to do it that way--you can create a more convincing review that's more likely to make sales. It's not always possible or practical, though; for example, would you break up with your significant other just to test a product for getting your ex back? ;-) In cases like that, or if the product is expensive, it's usually best just to use the vendor's affiliate resources instead.
This is important in the framework of not just affiliate marketing, but in marketing anything online, especially when the products are your own products and you're not just receiving a commission for selling them. Clearly, you need to build emotion around the entire process. In another words, you need to create an emotional attachment to every stage of the sales funnel.