I also have a 15" MacBook Pro, my iPhone 4 has just been replaced by iphone 5 and I bought an iPad 4 last week. I have predominantly used the MacBook for Internet browsing, emails, etc when at home, my iPhone whilst out and about and whenever I want to quickly check something at home.
Since buying the iPad, I haven't used the MacBook much other than for doing some write ups on my blog, I find the iPad easy to pick up and just browse. I use it for the Internet, emails, iMessage instead of sending from the iPhone, playing apps and checking social media. It's just so convenient to pick it up and put it down anywhere, in comparison the MacBook is just too large and cumbersome to do the same.
So in summary it's pretty much the only thing I use in the house and the iPhone does the job out of the house. Not sure if I need the MacBook now but having said that I still prefer a proper keyboard for lots of typing. I have wanted an iPad for ages but never thought I'd use it... turns out I do... A lot! The Mrs bought an iPad at the same time too and is glued to hers non stop. It's fast becoming an apple household! :-)