I have used fleece for a backing once and am planning on doing it again with a WIP however, the first time I was a very new quilter and I used it with a very low loft batting. The quilting I did was very minimal and it didn't turn out too bad...actually it is one of my favorite quilts and I use it quite often in the winter. It is about 4-5 yrs old and is about a double/full size.
Go ahead and try it...who knows...
Trying to get my WIP done so it is ready for this winter with the fleece and retire the other one since it is starting to show wear.
Kirsten