"Archaeology Of Knowledge" finds Foucault at his barest, trying to build up his own theory. Like others have said, it is fascinating to see how much he tries to encompass and how extremely difficult his own enterprise is. Foucault spends many pages trying to explain to us what he means by "discoursive formation", "object formation", "formation of concepts", etc., and the place where his own theory stands vis-à-vis a so-called "history of ideas". You can learn lots from this book, because, like myself, sometimes you get lost in Foucault's magistral writing, his fabulous way of weaving history and thus cannot clearly follow his own particular method of research. If you want to see some of his (earlier, almost stricly discourse-oriented) key concepts clarified, reading this book will prove very fruitful. As always, you're left with a lot of questions and with a distinctive feeling of "now what?". But then again, that's what's so utterly beautiful and engaging about Foucault... he forces you to think for yourself and provides you of the right tools to do it.
I read the spanish translation of this book so I can't comment on the english one, but the contents of this book are priceless.