While I agree with many of the iPad’s advantages Shawn lists, I think it greatly depends on what you mainly use your computer for if you could get by (mostly) using the iPad. I know I couldn’t. Many of the things I do with a computer either aren’t possible on the iPad or they’re cumbersome.
For me, the best creative use for my iPad is for writing. I like the focussed environment iOS puts me in and I like the writing apps that don’t dump hundreds features I don’t need on me. But even for writing I need an external keyboard, the virtual keyboard is hardly more than a crutch.