I vote: Meh.
My least favorite part about it, actually, is that in three years freelancers are going to re-retcon everything back into it anyway, except we'll still have demons as corrupted elementals and stuff. So by the "end" of 4.0 we'll have a confusing mis-mash that tries to please everyone but ends up just as confusing as what we have now.
Mark my words.
I'm also a little concerned about what happens to "scheming" demon lords like Graz'zt and Malcanthet. It's ironic that the Demonomicon is probably the most popular thing to hit Dragon in a decade and they're already making major changes to the popular ideas introduced in it. That's unfortunate, but I can see why they're doing it.
It's also unclear to what degree the planes will be akin to an afterlife, and the role of mortal souls on the planes remains unclear. I really (really) liked the idea that some demons were the "evolved" souls of wicked mortals (this is Orcus's origin, after all), and it seems like we may be losing that.
As a designer, I can see how to take these elements, weave them into the existing lore of the game, and come up with something that is both interesting and new and true to the tradition of the game. The problem is I just did that last year in a reasonably popular hardcover, and I'm not all that interested in doing it again either in a published product or in my own campaigns.
So while I don't hate the change, I grow increasingly wary of fluff changes that take the game away from the tradition I've enjoyed my whole life.
--Erik