I have a Kindle 3, and while its not as large as the DX, I'd say it was still a good alternative to the real thing, but it depends a lot on how you want to read the book.

If you read the book from cover to cover then it'll be great. If you want to look up specifics in the book, then it'll be great for that too thanks to the search.

On the other hand, if you want to flick through the book reading the parts that interest you then paper is still much easier to use.

As for an iPad, I can't see how it would possible be any better than a dedicated ebook reader for reading programming books. I'd stick with my Kindle.