I bought a "Mondo Bag" pattern that used gridded fusible to serve as the pattern. I bought some of those mini-charms that are 2-1/2" square and those were perfect for the project.
It was bulky and in the end I think I'd have rather just had a regular pattern and done the patchwork on my own - I could have strip-pieced it much faster. Plus now that I've made it, the pattern is gone, the only pattern was the fusible itself. But in the end I have a nice big bag that is put together nice and solid and it looks pretty darn good, if I do say so myself. No exposed raw edges on mine, at least!
I think the key to having it come out right is to clip aggressively on the first set of seams. Since you'll be seaming it the other way, it's OK to cut through the first seams stitches. (Assuming you stitch the same way this pattern dictated - all horizontal seams first, clip, then do all vertical seams). If you don't clip far enough, it gets really bunched up.