I haven't done any mystery quilts, possibly because I want to know what I'm making before I start it. It may not turn out the way I envisioned it at first, but at least I know where I think I'm headed. That said, I think one of the best reasons to do a mystery quilt, or any other BOM/quilt-along, is to be challenged each month to learn something new, step outside your boundaries, do something you wouldn't ordinarily do. And what's incredibly hard the first time will be easier the next time, and still easier the next, and finally it will become part of your skill set.