If you are starting out in IM, then this is definitely a good way to start. It's the ultimate in pre-selling - you write the review, get the person interested in the product and then send them somewhere where they can buy it. It is a very good way to gain experience and you can put the things you learn into practice with other methods later on.
There are also tonnes of ebooks which will give you a lot of useful tips for doing things the right way - I'm not going to recommend anything else, because the recommendations already given here are a good start and there is no point overloading it.