Our chain has used them for years. We stopped using them at our stores as the ads were not really brining in any new business. I’d say at least 95% of the coupons were redeemed were just from existing customers who knew we advertised there. That’s what your POS database is for.
I don’t know what your advertising budget is, but when we were part of another restaurant group, we had success with their full color inserts.
If you have never tried the Pennysaver I would suggest you try it. Its very inexpensive so the risk is nominal. I would not go less than a full page as I think anything less is easy to overlook.