It's useful for long highway trips. I don't usually use it but it's also nice to pop on (when safe to of course) so I can move my foot around and pop the joints if it starts cramping up on longer hauls. It is useful especially as it has button acceleration and deceleration (usually it's just acceleration).
If you're setting it to 9mph over the speed limit... it's your fault if you get caught because it went over. Cruise control is meant to keep you near the same speed, but it still has to compensate for varying loads and adjust to compensate.