Hmm. I agree with the sentiment that books are easier to browse through, it depends on the book on whether I'd rather have it over just DDI access.
Since I started using DDI in the past week, I've canceled any plans to buy any of the Powers books. I just don't see the point since the only reason I wanted them was for more character building options (although I must admit divine sanction is pretty baffling without it, I haven't been able to find clear rules on it). And it's not like I play a ton of characters, so if I got Martial Power it would be just for the one fighter I'm playing and so I'd be using, what like 5% of it (counting just the powers I'd end up taking?). Although I may still pick up the Primal Power book mainly because I really like all the classes in the power source, and I'm just a big fan of the power source as a whole. It still depends on what else is in there outside of character build options.
I'll still absolutely buy the PHBs though, because there's good discussion about the classes and power source, as well as additional rules, not to mention it being easier to browse through the new classes and read up on them all easily. And the DMGs since the DMG is already almost entirely text and discussion. I'm not sure about the monster manual. On the one hand, the art is nice. But I don't really use the lore information, nor do I really pay attention to the tactics. Also, what I really need is the art to be digital so I can make my battle map tokens, and DDI does not give me this. At least with the book I have the option to scan it in. I find the monster builder is easier to use than the MM index. I always flip to the back and look at the monster list by level, and the monster builder makes that even better. When MM3 comes out, I may not get it.
And all the other books I ignore, so the DDI is great in that it lets me access the crunch for those books I don't even pick up to look at in the store (and it's possible I might get sold on a book that way).