I also don't use cruise control. Not because I don't trust it, but I don't trust myself to react as quickly with it on. If my "gas foot" rests, it will take longer for it to hit the brake if needed, than if I engage continuously while driving. No scientific proof for this, except my own perception of my own attention.
Funny, I don't feel the same -- my foot rests below the brake (on the floor) instead of on the gas pedal so the time to move it to the brake feels roughly the same, and I'm paying 100% attention while driving since I'm still steering and constantly monitoring distance to the cars in front of me. So I don't notice any less attention -- it's just a rest for the muscles in my right foot.
But really I'd definitely go with your own perception here -- if you feel like you're paying less attention then it's a good thing you're not using it! Everyone's attention habits and patterns are different. And I'm glad to know people vary in this.
When I have tried to find a place for my foot when driving cruise control, it has happened that I got the toes of my foot stuck under the brake pedal. It was like I was resting the foot partly below it so when I moved it up, it was in the way. Can differ between cars surely. But in the cars I have driven with cruise control, I have not found a comfortable and at the same time "alert" location to put my foot.
I have the same worry when using cruise control. What I've trained myself to do is to cover the accelerator pedal when overtaking other cars or in any situation where I might have to brake. That way the car is still maintaining speed for me but I'm ready to intervene, to get back the safety margin. With adaptive cruise it may even increase the safety as both me and the cruise control can both brake.
I just set it to a speed on the highway... and the speed stays there until I brake.
Are you talking about adaptive cruise control (ACC) to follow the speed of the car in front?