Depends. If the reasoning is solid then I don't see why not. I've read some reviews on Amazon(US version) that were better than some blogs I've been to. The reviews for FFXIII were straight on the money in Japan before the US launch.
Amazon reviews do not require you to buy the product, allow reviews before the product is ever released, and has no filtering or moderation at all. Sure -some- opinions might be valid, but because the entire platform is flawed the time and effort it would take to consider the possibility of each entry's validity is pointless.