Let me state for the record that I absolutely hate the title of this module, which I think implicitly places far too much emphasis not just on the character of Mordenkainen but also on the adventure he and his companions had there in 1973. There are few people for whom the prehistory of Dungeons & Dragons is as important as it is for me. Had I the money and time to do so, I'd certainly be flying all over the country, interviewing people associated with the early days of the hobby in order to record their reminiscences for posterity. We've already lost far too many of our founding fathers as it is -- and their memories along with them. Their experiences at the start of it all are important and ought to be preserved.
Given that Mordenkainen's Fantastic Adventure contains no significant information about Gary Gygax's experiences in Rob Kuntz's Maure Castle dungeons -- for that you have to go to the May 1974 issue of Wargames Digest -- on which this module is based, I don't see a lot of point in drawing attention to his PC's exploration of it. Since it was published in 1984, I can't plausibly blame the Forgotten Realms or even Dragonlance as baleful influences over the marketing of module WG5. And given that the World of Greyhawk had (for the most part) been a setting largely free of treating NPCs as a focus of interest, there's something inappropriate -- and even unseemly -- about the title, at least for me. Consequently, I give points to the Paizo crew for giving it the much more straightforward title "Maure Castle" in their v.3.5 update in Dungeon #112.
With that grumble out of the way, let me now state for the record that I absolutely adore this module. I may not be fond of the proper name in the title, but WG5 is indeed a fantastic adventure. Consisting of three levels, Maure Castle is, in many ways, the Castle Greyhawk module we never got. Not literally, of course, since Maure Castle is a separate dungeon and originally was not even part of the Greyawk setting at all, but "spiritually" this module more clearly recalls the birth of the hobby and the early days of D&D than does, say, Gygax's own Castle Zagyg. Part of the reason I say that is WG5's presentation, which includes no boxed text, unlike many other contemporaneous modules (or Castle Zagyg). That may seem a small point, but it's not in my estimation. Mordenkainen's Fantastic Adventure is far less "canned" than most modules of the period (or since). Indeed, its room descriptions are quite terse, leaving a lot up to the referee to describe and adjudicate in play.
The content of the module is almost uniformly excellent, with a good mix of monster encounters, traps, and mysteries. It's exceptionally challenging, as one might expect of a module geared for 9th-12th level characters. Gygax himself, in his introduction, notes that "There is plenty of real thinking necessary, but the action is nearly non-stop," which I think is a good estimation of the adventure. It's definitely an old school adventure, one that felt like a bit of a throwback at the time of its publication. I know that it, along with several other late Gygax modules, seemed very much out of step with everything else TSR was publishing at the dawn of the Silver Age. In retrospect, I suspect that this was deliberate and I'm very grateful for its publication.
Its name aside, Mordenkainen's Fantastic Adventure is an important piece of gaming history: a record of one of an early D&D adventure and a solid example of what guys like Gary Gygax and Rob Kuntz thought dungeons should be like. A pity TSR didn't publish more modules like this.