I just started a 3.5 campaign, and I'm not switching that. I'll take a look at the 4e books, but almost certainly won't buy them. I might lift a few ideas as I did from Modern and various other sources, but I'm sticking with 3.5. I've got a ton of 3.5 stuff, and I like it.
Not only that, the first words on the new mechanics look horrible. They're talking about making fighters more like mages. I'd rather see mages be more like fighters (i. e. warlock). And with the supposed emphasis on "power centers", why no core psionics? And I actually kind of dislike the idea of making character prep simpler. I like spending hours on prep. It's fun!