I expect it's less of a problem on the iOS app store due to the lack of hardware diversity, but in the Android Market a lot of 1-star reviews are "DIDNT WORK ON MY PHONE IT SUX" which may or may not be useful - perhaps it really doesn't work on that model, but I've encountered apps that "don't work" on the model I have but which work fine on my phone.
The 5-star system is generally useless, though, with top ratings given to bad products merely because they function (this $10 PSU will probably fail next week and fry everything it's connected to, but it turned on! Five stars!), and bottom ones given out because of supply issues that had nothing to do with the product itself, or because that user happened to get one that was DOA and they had to get it replaced, or because they've never used the product but are biased against that brand.