I have an 8 Core 2.26Ghz Mac Pro with 24GB RAM, a Quad core 2.3Ghz i7 MacBook Pro with 8GB RAM (Antiglare screen) and a 16GB iPad 2.
My Mac Pro is the most cherished of them and my 'serious' Mac.
I use it mainly for creating music using Logic and also dabble in occasional video and Photoshop work.
However, the MacBook Pro is my 'main' computer and the one I use on a daily basis. I do about 70% off my stuff on it. It's incredibly powerful for a laptop and can accomplish most of the tasks that my Mac Pro would be used for. It's portability means it goes with me everywhere and it runs demanding applications like Photoshop with ease.
That said, on a recent side by side video file conversion, my Mac Pro still beat the crap out of the MacBook Pro (5 times faster) and The extra HD's, Optical drive (Blu Ray writer) and expansion PCIe slots (currently housing extra FW400 & USB2 ports), make the Mac Pro 'The Daddy' and irreplaceable for me.
The iPad is okay, but really just a bigger version of my iPhone. If it doesn't fit in your pocket, it may as well be a laptop in my opinion!
My wife likes it though. She uses it for Facebook, checking emails and BBC iPlayer (I do that on my MacBook Pro). I use the iPad mainly for playing games. Fifa 12, Real Racing HD, Need For Speed Hot Pursuit and Scrabble are all favourites!

To break it down I'd say 70% of my use is MacBook Pro, 28% is my Mac Pro and 2% is the iPad.