I do read reviews on Amazon.com even if I am going to buy the item in a different store. Sometimes I even read them after I have bought a product, and these are for motivational purposes to see what results I might expect.
Now, things I buy are not things like pills and such but usually exercise products, dvds, equipment, ect. I try to decide if the item might be in my fitness range and not too difficult or easy for me. If it is exercise for inside, I try to gather information on how much space I might need to use this product or dvd. If I will be jumping up and down alot and might not have the room for it. If it is a dvd, I will read the reviews as well as try to find youtube trailers of scenes from it.
Typically I read reviews for dvds, sports equipment like bikes ect (though I have not bought one yet) and I just bought a heart rate monitor and was reading reviews on it.
I have bought a Total Gym at Kmart and spent almost a year researching it before buying it. It is the most expensive item I have bought to date, and was definitely not a spure of them moment purchase.