Excellent question. I usually make my own patterns - I do simple quilts. However, for a friends first child I bought a pattern because the quilt was adorable, I could not 'do it in my head' and the Mom to be would love it.
What a poorly written pattern! It took me about twice as long as it should have and I wound up wasting a lot of fabric by cutting to the directions. I did finish it, and it is cute, but I will never buy a pattern from that designer again. The conclusion that I reached is that some people are excellent designers, but stink at writing directions.
Yellow Brick Road - not the one I'm talking about above - was poorly written, too. It took me through a section of piecing and then basically said, "now do the rest." Those weren't the official words, but... That quilt was donated, un-sewn, to a scrap quilter. And yet so many people I know love the pattern! To be fair, this was at the beginning of my quilting saga, I am color blind, the quilt instructions said 18 various fat quarters. I learned one other thing from that adventure - use a max of 5 fabrics in a quilt.