I tried them awhile ago... on a doll quilt...but, I used them at least 4 times before sewing on them.
I am not currently using them, due to all the info on the board...and I agree...you can just use the cheapest interfacing you can find or I have found a stash of flannel blankets at my Aunt's house and took a stained one home.
After washing and drying I cut it up into 10 1/2" squares. I avoided the stained area and tossed that.
It looks like this was a hospital blanket, as it had the hospital name in the edging.