I have a IP4, iPad and 17" MacBook Pro. Each is for very different purpose.
1. iPhone: goes everywhere and does some light web browsing, app usage, etc. Wonderful device but is weak for viewing movies and reading books.
2. MBP: I'm a photographer and use this for image editing, digital asset management, etc. It also manages my iTunes media. I could not get by without this.
3. iPad: Purchased for when I travel for my day job and want a personal computer as I don't put personal things on my work laptop. I use it in this capacity for viewing movies including Netflix, reading books (having the iPad has provided me the ability to discover reading again as I don't like dragging the books and magazines I would want to bring). I also use it for my kids as a quiz machine for math, etc. as well as some lighter web browsing.
They all compliment each other. I do not use the iPad everyday, but I use it enough where I find immense benefit in owning it. When I travel it is a life saver.
Just my experience.