I buy a ton of books and DVDs, and whenever I hear of a new title, the first thing I do is go to Amazon and read a handful of reviews. With a little experience, that can make the "success rate" of buying very high indeed.
One hitch to it is that old classics usually have much better reviews than recent hits, because only those who are really interested actually find them and buy them!
The same is true for genre products. For instance I bought a couple of horror films and was disappointed. Then I realized that all those five-star reviews came from people who love horror, and I only have a passing interest.