I like the convenience of a kit--especially if it is a complex pattern. BUT I have also been burned. One kit, bought for my DGD's first big girl bed, was short nearly a meter in fabric and I had to redesign the pattern to compensate. I never got any reply when I emailed the company with my complaints.
Now, the first thing I do, is open the kit, measure all the pieces and compare that to the checklist, even if I am not going to make the quilt right away.