The 4-6" ones are annoying bait stealers, or at best bait themselves for bigger fish (if you didn't have the huge shad supply on Kerr, they would be very good striper and cat bait).
If you can find them in the 9" plus length, they are good eating, although as far as I am concerned their thicker rib bones make them a little harder to clean than other pan fish.
If nothing else is biting, they can at least give you some action on the water.
I agree with the majority on this one. They are fun to catch especially the bigger ones. They are delicious and make great catfish bait. And like fd said if they get over populated can be a nuisance. So don't hesitate to take smaller ones out especially for bait.