I usually never use these because Sony is always higher priced than Kobo, so if I have a coupon at Kobo, I can get it cheaper than at Sony with Sony's coupon. I actually found a book (a Doctor Who book) I was able to apply it to that was cheaper at Sony (45% off list) than Kobo (28% off list)! Not sure if it was on sale at Sony or not, but I'll have to check the next time they issue a coupon... No telling when that will happen.
Edit: The price difference also isn't helped by the fact that Sony charges sales tax and Kobo doesn't (at least in my state). In this case, it still worked out cheaper.