For the most part I prefer to pick out my own colors and fabrics and buy at least 1/2 yard more of each fabric in case of mistakes or changes in the layout or size of the quilt.
I've bought a few kits and had good and bad experiences. First one was when I first began quilting and I was happy with the colors and that it was precut but the fabrics were a little thin. Picked up a few precut kits over the years but was disappointed that I had just enough fabric to make the quilt. The two kits that I bought on sale from Keepsake Quilting had excellent fabrics and directions and enough leftovers in case of mistakes by me. They were not precut. I didn't purchase the backings from KQ as they were expensive and I pick up top brand wide backing fabrics at low prices whenever I'm in Lancaster PA. Just finished one from KQ, a huge log cabin block that measures 66 x 86. Will post a picture after I attach the binding. May in Jersey