Published in 1982, Shadows of Yog-Sothoth is the first separate adventure product produced for Chaosium's Call of Cthulhu roleplaying game. It's not the first adventure ever produced for the game; that honor goes to "The Haunted House," a sample scenario that's been included in the back of every edition of the game since 1981. Moreover, Shadows of Yog-Sothoth isn't a single adventure anyway but rather a collection of seven linked scenarios, each one building on the one that came before.
Shadows of Yog-Sothoth is thus an early example of the "adventure path" model so in vogue these days. The early adventures are (largely) playable independently of the others, but the later ones all assume the Investigators have completed the previous ones in order to make sense, unlike, for example, the Giant-Drow series, whose individual modules are all, well, modular. Of course, the explicit linking of seven adventures into "a global campaign to save mankind" is precisely what made this product so revolutionary in its day. Shadows of Yog-Sothoth was a complete, pre-scripted campaign in about 60 pages, giving the Keeper everything he needed to keep his players' busy for months. It's not hard to see why it was so well received by gamers and critics alike.
I know I loved it. I've probably used Shadows of Yog-Sothoth as the basis for a Call of Cthulhu campaign more times than I care to remember. This is partly the result of its being a complete campaign under two covers and partly because the campaign it offers is a terrific, globe-trotting romp from Boston to Scotland to Easter Island, filled with memorable antagonists, such as the immortal sorcerer Carl Stanford (whose name is a reference to CoC's creator Sandy Petersen, whose full name is Carl Sanford Joslyn Petersen). Along the way the Investigators get to take on a plethora of Mythos opponents, including not just Nyarlathotep but even Great Cthulhu himself. It's a heady mixture of elements that I found irresistible back in the day and pretty well defined my sense of what Call of Cthulhu was like as a game.
Nowadays, I don't think as highly of it. The individual adventures feel more than a little railroad-y in places and the overall feel of the thing has, in my opinion, more in common with pulp adventures than with H.P. Lovecraft's brooding cosmicism. Of course, for some, that's precisely the appeal and neither criticism is a fatal one. In my experience, most gamers prefer a large dollop of pulp in their CoC games and the scenarios are short and loosely written enough that their railroad tracks can easily be modified or removed entirely without doing much harm either to them or to the overall direction of the campaign.
Still, Shadows of Yog-Sothoth is an early effort and it shows. It's a "classic" product more in the sense that it came early enough in the history of Call of Cthulhu to provide a template that other products would follow and improve upon rather than being a product that still retains its power nearly three decades later. I have many fond memories of playing through it, but I don't have any desire to return to it now -- a good example of where my memories of a thing are better than the thing itself.