This is an issue that you can see Amazon and users taking seriously. If you notice Amazon allows you to meta moderate reviews, as well as comment on reviews. I have spotted a lot of fake reviews by reading comments on reviews, and I make it a point to comment on fake reviews as well providing my rationale for why I think it is a fake review. Just like on slashdot, community moderation is key here.
Amazon also provides an additional level of verification with the "Amazon verified purchase" where by Amazon lets you know that this reviewer at the very least did purchase this product from Amazon.
In the end reviews are like asking for advice. Often you get bad advice, often you get advice from people with hidden motives, and often you get good advice. Amazon's review system is just a representation of real life. Go anywhere on the web, or go call up some real estate agents/mortgage brokers and ask them "When is a good time to buy a house?" and the answer will almost invariably be "If you can afford it, _now_ is a good time to buy a house". They have a hidden agenda they want to push.
I am not singling out real estate agents though. They are doing their job. Dealing with people is dealing with hidden agendas. :)