Also, if a product is popular enough, odds are that even with a great product there'll be a small fraction of people who'll just happen to get a dud, while most of the rest will be satisfied.
Because of this, I tend to read both good and bad reviews, and give more weight to the longer reviews that go in to great detail about the product and why they rated it the way they did, with comparisons to other products being especially helpful. Also helpful are reviews by professionals who really know the field thoroughly. Unfortunately, such reviews are rare.
I also look at the ratio of good reviews to bad reviews, and look for products with many reviews, though on Amazon this strategy is not quite as effective these days as it once was. One of the big problems with Amazon's model is that it seems that most people buy the products shown on the first page or two of search results. Products on later pages might be just as good or even better, but they may never get bought or reviewed because they weren't on the first couple of pages.