I just used basting spray for the first time on a quilt that measures about 72 x 102 (supposed to be the size for a twin bedspread).
I put the backing down and tacked down all around the outside to make sure it was flat with no puckers. Then, with a window open slightly, I sprayed it with the basting spray. I then took the batting and laid it on top. I was afraid that it would stick too much and I wouldn't be able to adjust it, but it did really well. When I got the batting smooth, I sprayed it and did the quilt top.
One thing- when you spray, have an extra piece of material for the edges- makes it easier than cleaning excess spray off your floor (I was on carpet so I really wanted to prevent that).
I love quilt basting spray now, and I don't think I will EVER go back to pinning. So much quicker and easier, and I don't have all those safety pins in my way when I'm quilting.