i have some sort of cruise control installed in my RVR. was already in there when i brought the car.
it uses a sensor box which screws into the speedo output of the gearbox, then the speedo cable goes into the sensor. problem i had was, not knowing how long the cruise had been installed for (while by the looks) the additional angle it put on the speedo cable caused the cable to fray and fail (month ago).
Another issue is getting a cruise control unit that is compatible with boosted cars, as most units rely on vacum to operate. the normal vacum one is whats in my RVR, but when you hit a hill (and boost) it wont maintain the speed you set.
Still, it all works and its not something i would like not to have.