Hands down, Infernum is and seems fit to remain my favorite "demonic/diabolic" planar layout of all time. Even as a hardcore fan of the World Axis, D&D has never made its demons or its devils or any of its other myriad fiends as inherently interesting and gameable as this setting does. It was Mongoose Publishing's finest work. For the Book of the Damned in particular, this is primarily a crunch-focused book; if you're familiar with the D20 mechanics, you'll generally know what to expect going in, but the race-as-class system and the mutations system is definitely worth checking out. I found it most interesting for the lore. My one gripe is the Sorcery system, and this is purely a matter of personal taste; I like magic for its destructive and combat-capable elements, and Infernum's firm slotting of magic into purely utilitarian spellcraft works, but isn't personally very satisfying to me.

Haven't played it yet. But, have read up to 40 or so pages and so far I am very impressed. Good writing and art. Great concept. Well executed. I'm surprised I didn't hear about this one earlier.

I find this to be an interesting setting. I don't do much modern d20 (there are some elements of this) and usually stay away from games to closely based on history or popular mythology but I found the system interesting.

As a free download (at least that is how I got it), it is worth a good read through. I plan on getting a couple more of the books in the series to see how it all fits together.

This is a complete d20 System game set in an interesting setting based on classical Hell. The different demonic factions are interesting and inventive and provide ample hooks for both combat and political games, and the mutations and races allow for some effective and interesting characters. The PCs are expected to play demons who need iliaster (soul-power) to live, fallen angels who can use iliaster, or mortals that can produce it; this creates a nice tension. This should make for an interesting Evil campaign (while a Good campaign is possible, it's somewhat difficult), but would require some learning of the rules. What mars it is that the rules are not written as professionally in D&D, and that ultimately it is limited in scope, and requires using specific mechanics with a learning cost. It is a good setting, but in my opinion not a brilliant one nor brilliantly rendered into d20 (despite some creative ideas).