Relying on visitors for your site content is one of the oldest tricks in the web publisher's toolbox. There is no greater source for content than your own visitors and the webmasters of forums and review sites can attest to this.
However one of the problems with this type of content is moderating it, making sure the content posted on your sites is content you will allow and is useful.
Amazon was perhaps the first company to experiment in mob moderation. Very shortly in it's life Amazon deduced that displaying real user reviews near products would be a great way to drive sales, however they did not have the man power to moderate all these reviews and so they decided to give regular users tools to provide the moderation. Now when you go to Amazon you can click on whether or not you found a review helpful, or click a button that warns Amazon of inappropriate content. In this way Amazon ensures that the best reviews rise to the top of their list.
Now sure, providing all that power to users could result in abuse, but Amazon hit upon the principle that people are generally good, and even if there are a few bad apples there are enough good apples to keep things as they should be.
The next major company to experiment in letting the users drive was Google. Google's PageRank algorithm is basically mob moderation for the entire web. Lawrence Page & Sergey Brin realized that computers could not judge quality like a human, and that hiring humans to judge the quality of the ever expanding Internet would be impossible. So they devised a way of checking how sites reference each other to come up with a quality score. What they were doing was very similar to Amazon except they were doing it without any new effort on the part of website owners. At the most basic level what they were doing was simply polling webmasters about each individual web page on the Internet, and it worked.
Since that time most review sites adopted an Amazon like system of allowing visitors to both post and rate reviews, and all major search engines emulated Google's use of link popularity to derive a quality score.
The next major development in mod moderation though was the wiki. Directories like DMOZ had problems keeping up with submissions and dealing with editor abuse, people looked at that situation and others like it and realized that even with volunteer editors you don't have to pay trying to edit a large amount of content is extremely difficult. So the next major free content sites used the wiki model. A wiki is basically documentation or information of some sort that can be edited by everyone. All versions of the content are saved so that if a user messes things up another user can come back and revert the document to an older state. A wiki is built upon the same principle that Amazon hit on, the fact that people are generally good and the good people will keep the bad ones in line. Wikis are not without their problems, specifically in the realm of accuracy, however they do very well at keeping out spam and inappropriate content.
In it's most recent incarnation mob moderation can be seen at work at sites like Digg.com where users vote on news stories much in the way that they vote on reviews on Amazon.com. In effect Digg is a community news portal where the community decides what stories get top billing. With a setup like Digg there is no wondering about what the visitors want and what type of content to serve them, they do the serving themselves and links only rise to the top if they are deserving and so spam is kept at a minimum.
Allowing users to review your content or products is one of the easiest and cheapest ways to obtain new unique content. For many sites they require users to register first and this does cut down on spam and inappropriate content, however it also cuts down on the amount of submitted content. To get the most content you should allow anyone to post a review and then simply either moderate it for inappropriate content (perhaps passing it through an obscenity filter) or moderate for such content yourself.
Then though turn the content over to your users. You see eventually you will get a large number of reviews and the truly quality reviews may get buried below the generic or less than useful ones. So allow your users to not only review your content or your product, but to review the reviews as well. You can provide a numerical scale for rating reviews, but simply thumbs up and thumbs down buttons seem to work the best. Eventually the most useful reviews will rise to the top and thus be read the most and that will make your site just that much more useful.
The only precaution you really need to make will be limiting voting by IP address to prevent abuse, and that is accomplished easily enough.
Review sites are some of the easiest and potentially most profitable content websites to create. Adding this type of functionality to your review site will help take it to the next level content wise and should help you find even greater success.