The K40 is rated highly. I am sure it is a great unit. I sell it, but have no experience with it personally. I will however say the same thing about it as I would any radar detector.
If you are alone on a hwy, a detector will do you little good. By the time it goes off, you are already caught. If you are traveling a busy hwy, the radar shot may bounce off other cars and set off your detector. You slow down, the other cars don't, you win.