One nice thing about the D20 book is the information on doing a DnD CoC crossover game.
I think it's more than nice! In fact, I think it's one of the great strengths of d20 CoC, particularly if you already have Chaosium's CoC, and it opens up so many cool options:
- Playing on the Dark Side -- All Clerics are insane cultists of Dark Gods.
- Playing on the Not-So-Dark Side -- The Cthulhu mythos is a pantheon of Dark Gods in an otherwise normal campaign.
- Pulp Sword & Sorcery -- There are no D&D spellcasters. Use the CoC magic rules (with Sanity loss and Ability Drain) instead.