i drove to tennessee from wisconsin twice and to denver n back:
i highly recommend cruise control when you're on those roads with practically nobody on it. those country highways where you see people once every 40 minutes.
i use cruise control only when im on a long stretch of road and there's not that much traffic. feels good to rest your right foot and keep the car going. otherwise, anything can happen with the drivers next to me and i always want to be cautious and ready to slow down/swerve/whatever if stuff hits the fan
cruise control automatically turns off when you apply the brake. it will stay on when i accelerate but i think it goes back down to what it was set at when i let off. typically theres buttons to fine-tune your cruise control speed.
just be careful when you use cruise control because you will be more relaxed and more than likely, less aware with whats going on around you.