1985's Isle of the Ape is the last official D&D module by Gary Gygax published by TSR. That alone makes it fairly significant. Like its predecessor in the WG series, Mordenkainen's Fantastic Adventure, this module is, at least in part, a throwback to the early days of the hobby, since the eponymous Isle was a feature of Gary's own (still-unpublished at that time) Castle Greyhawk. Though I have no firsthand knowledge to substantiate this supposition, I believe that these late Gygax modules were to some degree a reaction to the Hickman Revolution. I base my feelings on contemporary articles by Gary in Dragon, in which he makes the case that D&D had become too focused on "story" and that there was a need to "start pushing the pendulum the other way" back toward "action, rather than role playing, ... [as] the major focus of gaming." (Issue 102) Isle of the Ape has only the thinnest plot, being mostly a romp through an extremely deadly version of King Kong's Skull Island, re-imagined as a demiplane, so, if I'm wrong in this supposition, I don't do so without cause.
Remember too that 1985 is part of Gygax's Cent-Jours, after he wrested control of TSR back from the Blumes, but before he in turn lost control of the company to Lorraine Williams. By most accounts, TSR was deep in debt and on the verge of bankruptcy. In fairly short order, Gygax turned the company around, shedding personnel and projects that were both unnecessary and distracted the company from its core competencies. Books like Unearthed Arcana and Oriental Adventures were products of this time, being rushed out the door in order to generate revenue to cover TSR's financial obligations to creditors. So too were later Greyhawk products like this one. Written by Gary himself and harkening back to the early days of the hobby, these modules were less focused on a satisfying dramatic narrative or even world-building, preferring to concentrate on presenting exciting, action-oriented locales after the fashion of D&D's Golden Age modules.
Yet, in a sense, modules like Isle of the Ape represents a departure from the Golden Age's approach, even if not quite as large a one as does something like Dragonlance. WG6, for example, is explicitly written with the assumption that the referee owns and uses Unearthed Arcana, as there are unexplained references to magic items, classes, and rules from that "unearthly tome" (as the Introduction calls it). Certainly the module can be used profitably without UA, but the fact that the module assumes one is using a tertiary rulebook represents a very big shift in the way modules were written. It likewise marks one of several starting points for the "everything is core" movement that has, in my opinion, done great violence to the hobby.
Nevertheless, Isle of the Ape is a very interesting and enjoyable module. Written for characters of very high-level (18+), it's quite demanding and requires the use of equal parts magic, brute force, and wits to be able to overcome its challenges, most notably the giant ape Oonga, from whom it derives its name. The Isle is a mini-sandbox, with many interesting encounters throughout, most of them obviously derived from the Lost World genre of pulp fantasy. Gygax notes in the Foreword that the original Isle proved too difficult for his players, who opted "to risk other fell places rather than continuing to face the perils of the island." If the original was anything like the published version, I can believe it. When I ran it for the mightiest PCs of my old campaign, they also found it more than enough of a challenge. Indeed, some spoke of it as being worse than The Tomb of Horrors, because they all expected The Tomb to be a deathtrap, whereas they initially just saw the Isle as The Isle of Dread, Mark II, which it most certainly is not.
I have a soft spot in my heart for Isle of the Ape and appreciate its almost "retro" feel, both in terms of its inspirations and the way it flew in the face of the conventions of the post-Dragonlance world. And, for good or ill, it's also a window into some of the plans Gary Gygax might have had for the future development of AD&D had he remained at the helm of TSR. Consequently, it feels far more like an artifact from a different age than do many other even older modules, which may explain why I like it in spite of its flaws.