I have only done one true mystery quilt, but I was very pleased with the results. The instructions clearly said to choose three fabrics that I liked and to make sure there was a light, medium, and dark. Like I said, I was very pleased. In fact, I liked the quilt so much that I bought two books of patterns for mystery quilts. Of course, then the end results weren't surprises to me, but I really like the fabric choice instructions and they have helped me select fabrics on my own now.
I did jokingly say that I can never really visualize how a quilt will turn out, so every quilt is a mystery to me. It is only partly a joke.
The design wall in my head has never really engaged yet, but I am always hopeful that it will soon.
Dina