For months, I've read the morning newspapers on the Kindle. I find it's more convenient, it's easier to read and it saves trees. And I'm not alone: On a commuter train into New York City last week I counted 17 people using the Kindle.
But the real sign I'm hooked is when I got a review copy of a really great book a few days ago in the mail and was disappointed I couldn't put it on my Kindle.