I first came across the All Music Guide when it was in its infancy during the early 1990s. I was buying a lot of CDs from one of the first online retailers (CDconnection.com), and they posted the first early AMG guide ratings. I still use cdconnection.com to this day. Thankfully they have resisted the 'style over substance' paradigm that keeps me away from all their successors, and their plain but completely functional website is a gateway to unparalled availablity and competitive pricing.
But back to the AMG. I paid relatively little attention to it for years. Then it launched itself with a nicely designed website (that they chose to change last year, to the website's detriment in my opinion), but more importantly than that, with a series of nicely written and well judged critical reviews. I now regard the AMG as the premier review site. Not only is it extremely comprehensive, but there are sharp minds at work - Steven Thomas Erlewine, Richie Unterberger, Dave Thompson, Cub Koda, Bruce Eder and many others - who provide a solidly knowledgable background to the subject. They tend to err on the tolerant side, perhaps being overly kind to bands that have been critically savaged by others, but on the whole this is a good thing. Many critically unpopular bands (e.g. Kiss, Chicago) have proven to be influential on musicians and styles that followed and deserve proper consideration.
In this way, the AMG closely resembles Szatmary's excellent book "Rockin' In Time", a comprehensive social history of rock and roll, which also gives a more historical vs. aesthetic account of rock music.
Although both sources make for reading that lacks the sheer joy of pithy and opinionated musical criticism, they do impel you to hear the music for yourself. There is no substitute for this.