This has been discussed before, but is always worth bringing up periodically as there may be new review sites popping up here and there that few of us are aware of.
The problem with reviews online (or anywhere else, for that matter) is that you don't always know the standards being used to judge various aspects of the kit - if the reviewer is a nitpicker over instruction details ,f or example, but doesn't mind too much about using some putty to make imperfect seams meet; they usually focus on a few aspects of the kit and don't necessarily shed any light on other aspects that you might find important. That's unfortunate but true, given human nature. At any rate, here are a few review sites I refer to now and then:
http://www.hyperscale.com/ armor, aircraft, ships
http://modelingmadness.com/ armor, aircraft, ships, figures, cars
http://www.steelnavy.com/ navy, eh? an unbelievable online resource for ship modelers