I've played D&D since I was 8 years old. I'm 31 now. I will buy the 3 core books. For sure, no matter what. I will likely not purchase a single other book put out for it. Just as I haven't for 3.0 (only the 3 core) or 3.5 (still don't own a single 3.5 book).
I buy these because I play but mostly I buy them to see what's going on in the game I grew up playing. Less time to play, but with some of the announcements and rumors floating around, I think I'd like to make more time.
I still play other games (mostly M&M) but I utterly hate the idea of MMORPGs. If I purchase a game, that's it, I shouldn't have to shell out monthly bucks to play it as well. Just me. Don't bother flaming, I don't care.
I'm split on the D&DI though. I can see it taking the MMO track, but I can also see it being useful help me play D&D with some old friends. I highly doubt I'm going to convince them to not only purchase the physical books, but the D&DI subscription, plus the electronic copies of the books, just so we can all play the game online. Especially with other, free programs, that basically do the same thing. Sure, no minis, but hey, it's all about the imagination.
All in all, I'm really looking forward to 4th Edition.