I'm torn. When I started up 4e, last January, I wasn't sure we were going to stick with it and figured the DDi was the cheaper, easier route for a 3 month trial. Even after that, I expected not to buy many books because everything is in the DDi/Character Builder.
Now, I'm finding there are a few things for which one needs the books. Familiars and animal companions, for example. Airship combat, for another. There are also some cases where I don't want to accept the errata -- Rageblood Fighter, which I told my player was her option to errata the existing character or not -- but there is no way to get the original text from DDi, to my knowledge. For those things, you have to have the books.
Plus, even though I use a laptop at the table, it's much easier for me to lay out multiple books to different pages than to flip between screens on the computer.