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Bear in mind that most (all?) review sites are iTunes affiliates, meaning they benefit financially when you click an App Store link on their site. This is part of the reason you see far more positive reviews than negative - if they tell you an app is bad, you won't buy it and they in turn will get no money from you.
What I generally do is Google apps I'm interested in (usually after seeing a review or hearing about them in forums like this one), read a handful of reviews, and then open up iTunes and manually search for the app there (and read the reviews). It's amazing how the iTunes reviews left for the most part by people with no ulterior motives tends to differ from reviews on App review sites.