I have a 15" MBP and iPhone, and I wouldn't consider buying an iPad. Not at this price, anyway.
I can see their usefulness of an iPad for some people, but I know I wouldn't bother carrying the iPad around in my man-bag when I already have the much-more-convenient iPhone in my pocket for checking email, Facebook, and playing Sudoku. 😉
Also, I have a Kindle DX2, so my needs for an eBook reader are already met. Plus it's better for the task.
An iPad doesn't really fill any of my needs. It would be convenient if I were sitting on the couch, watching TV, and surfing the web. However, do I really want to spend more money on another device to check Facebook? It's certainly not designed to be a real computer, so I don't know what else I'd do with it. 😕