They are basically four stroke dirt bikes made to be street legal.
Think something like a Honda XR with lights and DOT tires.
They are a compromise, they sure don't handle like a sport bike, but you can't ride a sport bike down an occasional trail.
Not the big ones (IE 650s). The fast ones like the Ducati Hyermotard and others setup supermotard style are a real gas to ride. You can take them out and blitz back roads, even dirt roads, and when your in town you could hop an occasional curb if need be, stuff you could never do with a full faired sportbike.