While I've not used it as backing, I've used fleece as batting a couple of times when I've needed to produce an ultra-warm quilt. It's worked beautifully.
While some think fleece is too expensive to use as batting, remember that because it's encased between the top and the backing, you can use the cheapest stuff available. (I have a "Mill End Textiles" near me where I bought a remnant at ~$3/yd.)