I have been loving every minute of Castle Zagyg: The Upper Works since I received it for Christmas. While not a perfect product, I do believe this is a superior one. It is a great blend of accessibility and depth, a big boxed set that you can still wrap your head around. There's plenty of Gygax's signature whimsy, plenty of varied encounters (from the puzzle-solving to the hack n' slash to the "run away!" type), and plenty of nods to the history of Greyhawk and this hobby without turning it into a mere nostalgic journey. There's also plenty of easter eggs and open-ended opportunities to other adventures, for folks who enjoy those features.
I had high expectations for this module; I believe many of us did. And it can be hard to take an objective look at something that many folks will doubtless see through sentimental eyes. But there can be little doubt that Jeff Talanian, working with Gary Gygax, hit this one out of the park. I am supremely impressed with the writing, with the NPCs, with the encounters, and with the overall descriptions. The production values aren't supreme, but are clean, usable, and do not hinder the product in any way--at least a "B".
This puppy is available for all of two more days, thanks to the licensing issues. If you have the jack to do it and are at all a fan of Gygaxian dungeon crawlin' adventure, do yourself a service and pick it up while you still can. Whether you play Castles & Crusades, 3.5, OSRIC, Rules Cyclopedia D&D, or Labyrinth Lord. If you don't want to take my word for it, here's a list of other reviews I've found:
Allan Grohe (15-page review direct pdf link)