I have only once used a pattern out of a book, and I modified it even so. And it was a big pain in the rear, after I cut it all out I realized I could have assembled the blocks a little differently and saved myself a LOT of effort and probably used less fabric too. Definitely was a learning experience!
I mostly design my own patterns; either completely from my imagination or (more often) based either loosely or closely on something I saw in a book or magazine. I have EQ7 and between that and my trusty pad of graph paper I like to start off by reproducing the quilt as I saw it, and then I start moving things around, chopping things up, adding other stuff in... I have a lot of fun with the design phase, actually - it might be my favorite part of quilting!