Automatic Gratuity

How do yall feel about restaurants who add an automatic gratuity. Where I live most restaurants add a 15% gratuity no matter what...Lately I've noticed service at the restaurants have been terrible, even when not busy. Drinks running out without a refill, waiting for silverware and waiters/waitresses not checking up on us. Now I always tip but in instances like this I don't want to tip 15% for horrible service. I feel that the workers are just getting by with bejng mediocre because either way the are receiving a 15% tip anyways. How do yall feel? I feel restaurants Should give us the option of what we want to tip based off of service received.