O.K., let me try
I don't like Cook's Ranger. The Books of Eldritch Might caused a mixed reaction in me. Just so you know I'm not Monte-Fanboy.
I got interested in AU by Monte's design diaries (which you could read as a starting point, they're on his website). A player in my group, too.
Two other players weren't interested at all, and the last one was sceptical.
I bought AU, and it was the first rulebook in a long time that I actually read cover to cover in a very short time - including the spell descriptions.
I started a one-shot adventure with my group, just to see whether they like it.
Now, one player doesn't care whether we play D&D or AU, and the other three players are juggling character concepts and send me e-mails about an AU campaign. That's how well it was received.
I think AU is great for several reasons:
* The lack of alignments is a great improvement, causing more varied character reactions to situations as far as I have experienced.
* The classes and races are refreshingly new, and still incormporate well-known fantasy archetypes, just different from D&D (or, in some cases, the same types, but better solved).
* The magic system is unbelievably flexible and still very much D&D - it's simply wonderful and, imo, superior in all counts to the original D&D system.
* Flexibility and choice are promoted throughout. There's not only the standard "It's your game, change what you don't like", but actual discussion how to change things you don't like. The book doesn't force anyone to take an ECL race, but instead (with voluntary racial levels) includes the option to take one nonetheless.
* Put the power back into the hands of the DM. That's a big sentence, but somehow I think AU is succesful in that. Throughout the book, I really got the feeling that Monte gave me the bones of a great system, handing it over to me with the knowlewdge that I could - and would - bring its flesh and bones myself. The same with the campaign setting (Diamond Throne).
AU is simply marvelous, and a quantum leap in D&D with regards to the experienced roleplayer, one who knows how to interpret rules and knows basically how to play, and now wants flavorful options to make the game really great.
Yeah, I love it! It's as close to perfect as I have seen in D&D (and RPGs, in general).
Then again, I can be a little gushing with praise (and damnation)
Berandor