Based loosely on a tournament scenario from Origins II in 1976 that combined elements from Metamorphosis Alpha and a portion of Gary Gygax's Castle Greyhawk, 1980's Expedition to the Barrier Peaks is presented as "an exciting insertion into your campaign and as a primer on how to combine 'science' into your fantasy roleplaying." Gamers have been arguing about it ever since. That's because the decision to include overt science fiction elements into what is ostensibly a fantasy adventure scenario is a contentious one. One of the fault lines that rumbles beneath the surface of the hobby is the lack of distinction between fantasy, horror, and science fiction, three now-separate genres that had, prior to the 70s (if not later), peacefully co-existed as part of an indistinguishable mass of literature. Early D&D arose in such an environment and is pretty comfortable with such "genre bending," because early gamers (mostly) saw it as part of a long tradition, going at least as far back as classics like 1933's "The Tower of the Elephant" by Robert E. Howard. Gamers who grew up later or who were never immersed in the world of early "fantasy" fandom tend to cavil at such easy mixing of elements, which they see as breaking with fantasy conventions.
I am certainly sympathetic to those who don't want chocolate in their peanut butter when it comes to fantasy gaming. I occupy a weird middle place in this dispute, because, while I had plenty of contact with the remnants of the old days of fantasy fandom, I wasn't part of it myself. Instinctively, I'm part of the camp that sees sci-fi and fantasy as two separate genres of imaginative fiction. I'm also hyper-rationalist and prefer that my settings "make sense," which is to say, that I can explain how and why everything works the way it does, even if my explanations resort to the fantastic to do so. Having spaceships and lasers in a setting with gods and magic takes some heavy lifting to explain; it can be done but it's often more work than I prefer to undertake, so I avoid it.
Nowadays, though, I have come round, perhaps not to a full bore appreciation of "gonzo" settings, but a better understanding of the hows and whys of what some might see as genre mixing. It's very hard, if you have any knowledge of the history of the RPG hobby and the fandoms from which it sprang, to get worked up about robots and aliens in Greyhawk. They've always been there, just as they've always been a part of weird fiction. The boxes we now use to categorize -- and market! -- our creative products are purely artificial, the result primarily of bean counters looking for ways to sell their wares more effectively. "Genre" nowadays is often more an exercise in brand building than literary theory and modules like S3 are throwbacks to the days before such a mindset was commonplace.
The module itself is effectively a dungeon crawl, but in a dungeon of steel and plastic rather than stone and mud. The crashed spaceship is large and filled with a wide variety of environments, making it a terrific set-up for encounters of many sorts. These encounters include many memorable new monsters, like the froghemoth and vegepygmies, and gives us hints into a possible interpretation of the illithids that I think works far better than anything we saw subsequently. The "magic items" of the module are technological artifacts whose use is potentially dangerous, thanks to a series of charts Gamma World fans should recognize. I'm also fond of the illustrations of these artifacts, very few of which look anything like you might expect, which gives them a genuinely alien and high-tech feel to them. Indeed, Expedition to the Barrier Peaks may be one of the most lavishly illustrated modules ever, since it came with a booklet containing 63 separate pieces of artwork, many by Erol Otus, who, as an artist, was probably destined to contribute to a product like this.
Like most modules of the period, there's only the thinnest outline of a plot and little in the way of context or explanation about the spaceship and its origins. Referees are thus left to their own devices to provide these things. Back in the day, I never did so, but I think S3 could, if the referee is willing, be the catalyst for some very fascinating and potentially setting-changing events. Dave Arneson's Blackmoor campaign -- almost certainly an inspiration for this module -- provides one example of how this might proceed and Paizo's Golarion includes a country called Numeria that answers much-debated question of what might have happened if Conan or Kull had had access to lasers and robots. Expedition to the Barrier Peaks may not be to everyone's tastes, especially nowadays, but it's nonetheless an excellent romp and a time capsule from an age before the demandsd of marketing narrowed our sense of what was and was not "fantasy." I re-read S3 every few months to remind myself of this; it's a practice I recommend highly if you're able to do so.