At all restaurants, even ones that are not Disney owned (like Blue Zoo, Shulas, Il Mulino, House of Blues, Bongos, Portobellos, and T Rex)? For some reason I can't seem to find this answer anywhere.
To: OP we've used them at Disney owned restaurants without a problem and usually carry one just to cover our tips if we are on the DDP.
We've also used one to buy an AP last Oct (in fact I used a combination of an old unused one day ticket, an old Disney gift card that had some money left on it, and some Disney Reward points that I cashed in just before our trip). I round up paying about .70 cash for my current AP.lol