First, a disclaimer: I'm a backer of the game from the KS campaign 2 years ago or so. No other connection to authors except occasional talk on Discord.
SINS is a curious case. It's a post-apocalyptic game, offering a standard package of devastated, zombie-infested world, closer to "the Walking Dead" than any zombie apocalypse settings (there are cultures, organizations, secret societies and religious orders). The catch is, using standard mode of play, you're essentially a zombie, not a generic shambler but ass-kicking monstrosity, also the one which remebers its humanity and strives to balance the human nature with monstrous urges. You also look pretty human, in most cases actually but not always. Seems familiar? Yup. It's part zombie post-apocalypse, part World of Darkness (Wraith: the Oblivion and Kindred of the East mostly), part BESM/OVA with a lot of anime stylistics and part Exalted, but don't tell the last to the authors, they don't like mentioning the last one :).
For such an ecclectic bunch the game is neat, and you don't see seams on this Frankensteinian monster of the setting. The main premise can be shortened to "everything you know, but different way". The world is devastated -- not by freak of nature or folly of human, but by planned extinction plan of some yet-to-be-revealed outer space entity (plan that gone south by the way, so at least the entity is not omnipotent). Zombies roam the wastes, but they are far from anything you could suspect -- they don't eat brains, human meat or anything at all, they simply seem to draw sustenance from slaughter and destruction. They also evolve into bigger and more horrific forms until they form gargantuan abomination (and everything is screwed then horribly). Characters are sort-of Exalts from the game you should not mention, not created by bengin gods, but by utterely alien entities, the by-product of botched extinction plan. They have kick-ass, anime-styled powers (including the ability of transforming into small-size mecha) but also they have a monstrous anti-soul that can sometimes turn them into dangerous monsters, and its agenda is to achieve the state permanently (you can scream "Wraith" but it's more "Kindred of the East"). Ah, also to become one you must die, turn into the zombie and then hope the rest will go smoothly (because only two first parts are foolproof). You can be killed, "diablerized" Vampire way or turned into even more monstrous zombie at the end, and there is no third chance anyway (dead protagonists are simply anihilated, they don't have any hope for afterlife, unlike more normal denizens of the setting). If you love ecclectic, baroque settings, you'll probably love this one.
Mechanics of the game is also familiar, yet different. It has a lot of Storyteller vibe - success based mechanics, health levels, skills and attributes and such. It is D6 based (instead of D10) and you roll only attributes, skills set target number to get successes. Difficulty is a minimum success level, and extra successes you can use to improve damage, get job faster and such. Combat is long-turn based (I haven't seen that approach for a long time) so the rolls are abstract, one hit roll can be translated into many exchanges of blows that all were parried except the last one, which made damage. It works well and didn't gave me too many headaches.
The book is nice and layout proffesional. The only thing I could criticize is a low number of artwork, all with distinctive but boring on the longer term style. Don't get me wrong, these are nice and proffesional graphics, but I'm accustomed to more variety in style, especially for a book that big (it's a standard monstrous "brick" of old, not these newer booklet-style, low page count games like many Powered by Apocalypse books, so while PbtA also are artwork-low one-artist productions, it's less problematic). There are glitches in the layout but they are surprisingly rare. Material is presented nicely, there are some worse moments (like presenting the mechanics for "normal people" first, where it's not essentially a game about "normal people"), but I've seen worse, and this is their first product.
You also should check "Dead City" scenario, but it surprisingly uses slightly different mechanics (newer? older?) and it's not about badass supers in post-apo society but about normal lame-ass random guys put in the middle of zombie outbreak in present time, but still it's neat and nicely connected to the main game.