CoC d20 is a very touchy subject in my group - we have a couple of guys who are big fans of the classic version and get all agitated when the subject of d20 Cthulhu comes up.
I think it's an excellent product - and it'd be ideal for running non-Lovecraftian horror games, like, say, Buffy or All Flesh Must be Eaten. (I much prefer it over Eden's Buffy RPG.) Nevertheless, I wouldn't run Lovecraftian horror with it - I prefer the simpler BRP system for that, since so much of what CoC is about is atmosphere, mood and the ultimately fragile nature of characters. I think those things are better conveyed to the players with the BRP system. In addition, I believe that the level system as it's used in baseline d20 (including D&D and CoC) is frankly incompatible with traditional CoC character design. (There's an easy fix for that, though - just set level at wherever you think the competency level of the PCs ought to be - and never give out experience again. Occasionally grant characters a skill rank or two instead.)
I've got both Dunwich and Arkham. Both are very well-done, but I agree with haiiro in that I don't think either book is an ideal first pickup for the Cthluhu neophyte. I'd say pick up some of the BRP adventures and convert them - it's easy enough to do on the fly (and if you're reluctant to do that, there's guidelines in the GM Pack) and they are usually very good adventures. I know one guy who ran the entire Beyond the Moutains of Madness campaign - twice - while converting it in-game as he went. It ain't hard.
