I have a custom built script I use for my sites which automates this but you could just as easily allow people to email you and add their review to the bottom of any page.
There are a butt load of advantages here.
- You get return visitors because people want to see if their review got published
- You can collect email addresses of people you KNOW have bought
- Fills out the content on the page with natural words, helps your rankings
- Gives your site more credibility because it shows you arent afraid to publish an opinion apart from your own.
Try it, works wonders.