Astonishing Tieflings does what any racial splatbook should do: it presents threads of ideas and evocative mechanics that make you want to go make a character. The editing is good but not perfect, with a few loose threads that slipped past editing but don't interfere terribly with the overall charm and success of the book. The layout meets the high bar set by other Fat Goblin products: highly readable, professional, and attractive, with six pieces of original art that more than justify the asking price.
The alternate racial traits and class archetypes are a strong spice that GMs should use carefully. Some are just fine, laterally trading racial abilities to help establish the tainted background of your tiefling--"Seeding the Air," for example (p 13), trades darkness for fog cloud, an even spell-level-for-spell-level switch that no storyteller would protest. A more unique ability swap is Cage of Ribs (p 10), which trades the minor 1 RP fiendish sorcery racial trait for a hollow lung-pocket in your ribs to use as a hiding place. Yet for the same price, Astonishing Tieflings offers "Beastly Brutality" (p 9), which increases the threat range of unarmed strikes to 19-20/x3! So too with the class archetypes, where Abyssal Brute grants +2 to hit and damage for an additional -2 to armor class and trades uncanny dodge for a scaling fire aura that deals multiple d6s as the barbarian levels.
While the mechanics are hit and miss, the fluff hits home every time. A tiefling subrace of sorts called "Croakers," for example, breed boggards and hezrou demons to create amphibious toad-like demons who wield oversized shields that double as basket boats. I was only halfway through the section when I started fantasizing about using Croakers to replace goblins as disposable mooks in my next game. There are many moments like this in Astonishing Tieflings, where the writing is so on point that you start playing D&D in your head mid-paragraph. Even the poorly balanced mechanics benefit from vivid description to the point that, if a player was absolutely seduced by a mechanic I disagreed with on a game design level, I'd probably still approve it in the name of story.
One final gripe. Amid all the excellent writing is a great deal of reprinting from the Advanced Race Guide. It's all OGL and copyright-legal, but nothing distinguishes the original Paizo content from Fat Goblin's contributions. Despite being a 35-page PDF, roughly half is a reprint, which devalues the product in my eyes as it pads the pagecount with borrowed content.
Overall, I am happy with my purchase. The writing and layout are excellent; the mechanics and editing are fair. I'd have preferred a different way to handle the reprinted Paizo text, but even so, there is enough quality writing here to make this worth a trip through the Drivethru.