My iPhone is always with me, it is my swiss army knife, jack of all trades portable communication device.
The iPad replaced my MacBook Pro as my travel and around the house casual computing tool.
I use others machines for heavy lifting, but the iPad does media consumption better (for me) than anything else. I suppose you could huddle around the iPhone for hrs a day, but I would not find it as enjoyable by a long shot.
My wife's MacBook was on its deathbed, and instead of replacing it with a MacBook Air, she decided to get an iPad. 99% of her computing is consumption, web, mail, social media - she could not be happier.
She already had an iPhone 4, but once you use the iPad you realize it is much more than 'a big iPod Touch'.
Everything comes alive, it's a book, it's holding the web in your hands, it's magazines and newspapers arriving for you before breakfast.