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).