The *Fiend Folio* has 166 creatures, 224 pages, and 8 templates, as well as 3 prestige classes for fiends and a section on grafts and symbionts (which is very cool, if not necessarily easy to implement).
Its CR values: mean ~8.3; mode 9; median 8; low 1/8; high 25.
CR values of *MM1 3E* for comparison: mean ~5.4; mode 3; median 5; low 1/10; high 26.
It retails at [$$$], which is a cost of $0.13 per page.
Overall, these values are close to *MM2*, and the focus of the text is true to its title, as over 50% of the entries are extraplanar in some fundamental way. If one is looking to enhance some kind of Blood War narrative or planewalking game, then this book would be highly useful, for it does round out the possibilities hinted at in the Manual of the Planes; it is not so obviously useful for a straightforward "let's jump in the pickup and git us some kobolds!" type of game.
Noteworthy templates include the "half-fey," "half-troll," and "half-illithid" (along with a cool sidebar on illithid reproductive activities heh). The "huecuva" and "swordwraith" templates are also decent.
Specific creatures worthy of comment:
1) Constructs such as the "blackstone gigant," the "caryatid column," the "necrophidius," and the "zodar" (this last is especially well rendered), along with two new "inevitables" of high CR.
2) Undead: the "crawling head" and the "crypt thing" are decent, and the "hullathoin" is good. My favorite is the "quth-maren," a mid-range beastie that is pretty much the skinless guy from *Hellraiser*. Very nice. (but where's Pinhead, beeyotch?)
3) Dragon: only 2, but they're alright-the "sunwyrm," which is decent, and the "sea drake," which is very well executed (it likes to charge tolls to ships that pass through its territory, and if they don't pay, it constricts the vessels and sinks them; it especially hates pirates).
4) Planar-stuff, which is the meat by which this text must be judged:
--the "rilmani" entries are a nice touch (creatures as adamant about true neutrality as the tanar'ri are about CE);
--the "nerra" (from the "Plane of Mirrors," an area mentioned in *MotP*, but not at all developed until this entry, which is pregnant with idea);
--the "kaorti" (from the "Far Realm," another area mentioned in *MotP*, which i interpret to be some space outside of the normal planes that is not subject to the axes of Good-Evil and Order-Chaos, and with plenty of other weirdness about it, etc);
--more "planetouched" types e.g., dwarf-demon combos, &c;
--the old 2E gehreleths return, but with their 1E name of "demodands," to round out Carceri's politics;
--a miscellany of cool bits: the "khaasta," some "slaad," more "formians," more low ranking "yugoloths" (much needed, since *MotP* & *MM2* give mostly tougher ones), and the "ethergaunts" (a malevolent culture on the Ethereal Plane--very well conceived, since before the Ethereal was an apolitical space with ghosts and other nasties that merely want to eat people);
--and of course tons of Celestials (2 "deva" types), Tanar'ri (7 or so types, some aquatic), and Ba'atezu (2 with high CR) plus allied creatures, like the "canomorphs," the "varrangoins," and some innovative golems.
5) A so-so assortment of oozes (the best is the "bloodbloater," a floating fried egg-looking thing that forms swarms to feed on the unwary), fey (some evil ones, too), plants, monstrous humanoids, beasts, and the like, though nothing about which to cream one's pant-except for:
--the developments herein to the yuan-ti, where we now have the "yuan-ti anathema," a huge nasty-nasty with no human bits at all, and 6 snakeheads instead-high high CR, as well as notes on the "ophidian," a yuan-ti slave race of humans morphed into snakey-things, the "ti-khana" template (yuan-ti breeding experiment), and of course tons of fairly sickening (and completely characteristic) yuan-ti grafts in the appendix;
--perhaps, also, the rules for swarms are useful, though the focus in the samples is clearly on tougher planar swarms, rather than on mundane types that (I believe) show up in *MM1 3.5E*;
--one last thing here of interest is the "sarkrith," a mid-to-high CR lizard-thing that hates magickes; the entirety of sarkrith society (based in the underdark) is intent on destroying magic and enslaving those who use it-a fine addition to any game's eco-political system.
6) There is, I should note, a perverse worm fetish here; I just can't understand why anyone would need tons of high CR worm-things that a) have no intelligence, and b) are useful only insofar as they bust out of the ground, swallow half a party, and then retreat (isn't there already a "purple worm" and a "frost worm" for this purpose in *MM1*, as well as a hellacool "fiend worm" in *MM2*?).
Anyway, *FF* offers the "century worm" (CR 19), the "lucent worm" (CR 17), the "slasrith" (CR 7-coolest one, since it is just a flying worm that yugoloths use as mounts, and has a useable CR), the "thunder worm" (CR 21), the "tunnel terror" (CR 15), and the "ulgurstasta" (CR 11-an undead worm-thing that swallows whole, like the rest of the lot, but instead of eating you, turns you into a zombie, then barfs you up hahahahaha); well, yeah, you get the point-should be the *Worm Folio*, &c.
Otherwise, the text has the same high production values-great art here (best drawing by far is for the "blood golem of hextor," which looks just like a Mad Cat mech (a gift to the battletech cross-pollination geeks, i guess), no obvious editorial errors, and for the most part 3.5 compatible, though the DR system needs to be updated with the WotC web enhancement. The cover art is 100% better than *MM1* or *MM2*, since it's obviously a view from a cavern-type area out onto some abyssal plane, complete with a pit, volcano, and fire-stuff shooting out everywhere, rather than the silly eyeball-thing on *MM1* or the braindead mouth-thing on *MM2*.
Ultimately, worth the money for game's that are not content to hunt orcs through level 30.