It already exsists. Twice.
http://www.saxperience.com/forum/articles.php?c=141 - Should be made open for all people to post in. But with it being on the main site there I reckon alot of thick people will give bad reviews of a product because of the place they bought it from giving bad service, or they will rate shit things a good because they dont know any better.
Also in the E-Garage there is the product section and you can rate and review your parts in there
If aother was to be made, they should have a template and include "similar products used" if theres a nice list of 5 alternatives then their review would be more reliable than someone who hasnt used any other. Abit like people thinkin Compbrake quickshifts are good and never used anything else.