I especially like the fusible when I'm quilting a top which someone has washed (and has fraying seams) or which doesn't lie flat due to less-than-perfect piecing. (usually mine! :oops: )
I don't use it for all quilts because it is kind of a pain in the neck to iron it to both sides, and it doesn't come in a king size. But it's perfect for problematic tops and for small quilts.
The fusible isn't sticky like the spray, but it does a good job of keeping your fabric flat and smooth. Fuse the front first, then the back. You may want to wash and iron the backing fabric beforehand, so that it's pre-shrunk and the sizing is washed out - the sizing may prevent the fusible from sticking as well as it can. (as might starch)
I've used mostly the Hobbs brand fusible, which feels stiff and decidedly un-cuddly when you're working with it, but it becomes much softer when you wash it and dry it in the dryer. :)
Edited to add: if you pin around the outside edges of the quilt, it will keep the layers from separating and helps maintain the hold from the fusible.