I realize that, in some quarters, that's a controversial, verging on blasphemous, opinion. There is a contingent of old schoolers for whom the Fiend Folio is the best monster book published for AD&D – alas, not for me. I owned it more out of completeness than any enthusiasm. I adored the Monster Manual, which was my first AD&D hardcover, ordered at a Sears catalog store with money given to me for Christmas by my grandmother. I still cherish that book to this day, a fond possession from my youth, portions of whose text I can quote from memory, so often did I read it in those early days. The Fiend Folio, though? I've barely cracked the spine.
I used to think, when the matter of the Fiend Folio came up in conversation, that my dislike of it was based on a failure to appreciate the book's idiosyncratic Britishness. The tome quite clearly evinces a different sensibility from its rather staid American predecessor, most notably in its illustrations. Though the volume contains artwork by TSR stalwarts like Jeff Dee, Erol Otus, and Dave Sutherland, their familiar visuals were buried beneath an avalanche of pieces by Alan Hunter, Albie Fiore, Russ Nicholson, and others, none of which looked much like what I'd seen in the Monster Manual. There was a gloomy, gritty quality to the illustrations that shocked and repulsed me at the time. This wasn't what Dungeons & Dragons was supposed to look like and I found it hard to accept.
But it wasn't (just) the artwork that turned me against the Fiend Folio; it was the content. Compared to the Monster Manual, most of the creatures in this book are, at best, weird and strangely specific and, at worst, downright silly. Again, I recognize that many see this as precisely why they like the Fiend Folio. I can see that, but, for me, monsters like the Enveloper, the Flail Snail, and the Gorbel, to cite a few obvious ones, are simply goofy and I can't think of any circumstance in which I'd use them. And they're not alone. I could easily go through the book, page after page, and point out all the monsters that strike me as too ridiculous (lava children), overpowered (death knight), or bizarre (trilloch) for my tastes. The whole thing has a rough, unfinished, and fannish quality to it – filled with the kinds of monsters overly enthusiastic but not very creative kids would come up with for their homebrew adventures. I realize that's an unduly harsh judgment, but it's how I felt at the time.
In the years since, my opinion of the Fiend Folio has softened a bit, in particular with regards to the art, some of which I now consider among the best ever done for AD&D. Russ Nicholson, for example, is now a favorite of mine and I am somewhat embarrassed to admit that I once didn't much care for his illustrations. Likewise, I've come to accept that there's a place for some of these monsters, if only as occasional palate cleansers. However, except for those that had appeared previously in published adventures (like the drow or the bullywugs), there aren't any that I feel fill an obvious gap in AD&D's roster of monstrous opponents. The Fiend Folio should, therefore, be treated as a book of options to be used with care rather than as a regular supplement to the Monster Manual (or Monster Manual II, which I consider a much better book, despite its flaws). Viewed in this fashion, I think of it much more kindly.
But before I forget, it must be said: the githyanki are overrated. Ugh.