I'd honestly rather read linear (read from page one to the end, one page at a time) fiction from it than a book, because I can't always get comfortable with a book. Hardcovers are sometimes a bit heavy, and paperbacks don't always lie open easily. The Kindle is incredibly light and thin. I can hold it in one hand easily. The page turn buttons are conveniently located. Page-turns aren't instant, but they're probably quicker than turning a physical page in a printed book (there are just a lot more page-turns unless you choose a small font). The contrast is better than other ereaders I've seen. There is zero eye strain in good light. My eyesight isn't the greatest and I like being able to increase the font size and read without glasses. I love being able to browse the Kindle store and read samples before deciding to purchase. The "experimental" browser is surprisingly usable, but isn't great. It is useful for browsing wikipedia and blogs. The biggest drawback to the browser is the awkward pointer navigation, using the 5-way pad. It syncs your furthest read page over the internet so you can pick up where you left off using your iPhone or iPad.
Apple iPad is a lot better for magazines, reference materials, and illustrated materials.
Kindle is worlds better for reading novels.
iPad is pretty heavy, making it more difficult to hold in your hand or carry with you everywhere. Kindle is much more portable and easier to hold.
iPad has some amazing children's books and magazines, which take advantage of its multimedia features.
iPad is unreadable in sunlight and glare is bad in bright light. Kindle is as good as a printed page in bright light.
iPad serves as a creative tool, a computing tool, a gaming tool, and a communication tool. Kindle is only a novel machine.
I don't regret buying either one of them. An iPad won't replace books, but a Kindle can, if the book is text-only.