I don't know how long Hell has been an integral part of D&D, but certainly it was with the first edition Monster Manual, which had a catalog of greater and lesser devils, and the archdevils Asmodeus, Beelzebub, Mephistopheles, etc. Hell in D&D, no matter who's treated it, has been strongly influenced by Dante's Inferno and every iteration of Hell published has nine "circles" of Hell, many of them bearing the exact names and descriptions of Dante's own, with others changed to be more suitable to the D&D cosmology.
This is true also of this book; unsurprisingly. One thing that Hell and the devils have not managed to do, is eclipse the demons. I guess the variety of demons makes them more attractive, perhaps. Their more independent nature. Maybe plain ole destruction is just an easier motivation for folks to latch onto than enslavement and damnation. Lots of folks have tried to take the devils and make them really interesting. Chris Pramas did a great job of it in Legions of Hell, although that was more a catalog of monsters with only a very short section on the diabolical cosmology itself. Robert Schwalb and Co. took on the Fiendish Codex 2: Tyrants of the Nine Hells where they made Hell into, basically, an analog of corporate America's cubicle culture. A cute idea, but probably one destined to ultimately fail.
Paizo's Wes Schneider gives the devils an almost Lovecraftian spin. According to the doctrine of the Church of Asmodeus (although plenty of hints here and there suggest that this isn't just one of his heresies), the Archdevil himself is the most primal god in the entire multiverse, and the rigid tyranny of Hell is the natural state of the universe. With a bit of secret history, with a bit of blasphemous meddling, terrible books that attack your sanity just to read, and with a few pages that look like they took their inspiration from Black Library's brilliant Liber Chaotica series (I only own the Khorne one, but I saw some clear parallels), the devil is finally given his due.
Most of the book is fluff; there's a lot of stuff about the arrangement and cosmology of Hell itself which is an interesting read, although not necessarily something you'd use in your everyday game. There is an interesting and challenging prestige class, that I'd love to see get some play in a game (it would sure put to bed such notions as D&D being a high fantasy only vehicle; this class has a very sword & sorcery feel to it.) It's got some spells, some monster entries, a few magic items... pretty much what you'd expect. But really, it's the cosmology that makes it interesting. The fact that it really takes devils, gives them an agenda that you feel you can work with, and some legs that you can use to get some real traction for a slightly different type of villainy in your campaigns.
Of course, this is also coming out at the same time (more or less) as the latest adventure path, which is set in a devil-worshipping city in Cheliax, and at the same time as the Cheliax book in the Pathfinder Companion series... so it's greater than just what's here in this book alone. Combining the three together gives you something that's greater than merely the sum of the parts.
Anyway, recommended. Paizo continues to impress me with their setting stuff. I'd often said that vanilla D&D didn't hold any more attraction for me, but somehow Paizo have managed to pull it off. With their Golarion setting, they've managed to make the old, tired, cliched even, feel fresh again. This book is the latest chapter in that same success story.