I've used muslin as backing on a couple of quilts. As far as performance, functionality, and looks, it's wonderful. However my tastes have changed and I now prefer a bit of color in my backs, especially if I can use orphan blocks or excess fabric. When I make stack-n-whack or OBWs, I make sure to put some of the uncut fabric on the back of the quilt. Makes it fun for the quilt recipient to compare the original fabric to the finished blocks.