Your cushions are to your pool table as what your tires are to you car. When they wear down your performance suffers. Let them go too long after they go bad and it will make play (or driving for that matter) poor if not dangerous.  Below are a few pictures of what happens when a cushion wears out.

There are a few different ways that a cushion can go bad; hardening, glue breakdown or the most dramatic of all disintegration. Since it is impossible to show a hard cushion in a picture you are gonna have to trust me on this one! You can test your cushions at home with a careful ear and by pinching the cushion where the ball hits. The rebound of the ball at moderate to fast speed should make the same sounds all around the table on each of the 6 rails. Each table will sound differently based on the design of the rails and frame resonating the energy from the ball striking the rubber cushion. You should also be able to squeeze the cushion with finger presure between 1/8" and 1/4". Sometimes the signs are not apparent until the cloth comes off, at which point your technician will explain the options available and costs involved. Typically cushions only have a life of 5-10 years, but certain brands will vary greatly. Once you know the cushions are bad, you should plan ahead to change them at the same time you have new cloth installed. Nothing is worse than spending good money on a pool table, just to have the dead cushions making play miserable. This is to be considered a normal maintenance item, though few new table manufacturers will tell you that. You wouldn't drive on the highway with your family in the car if you knew you had bald tires, why take the play out of your playroom?