I always love the "it's just like physical property! Except that of course I can sell it 5000 times, and you can't even though you have the same ability to copy it, and the laws about who has the right to copy it can change when you cross national boundaries! And it's okay to resell pbook copies but not ebook copies, not even if you sell the whole device, because you might have made another copy!"

The DRM comparison I've liked is "DRM is like saying you have to use our special glasses to read our books."