Just FYI, pure butter will keep at room temperature as long as you keep it covered. I have been doing this for years. I do not think it works with margarine.
I do the same thing, Brian. But, I have found that my butter is a lot firmer in the winter because I keep my house a lot colder in the winter to try to save on electricity and a lot warmer in the summer for the same reason. In the summer I have no problems spreading my butter.
But, in the winter it would be nice to have one of these new-fangled gadgets.
I don't leave margarine out, though.