I have one of the genuine volvo accessory ones (although made by Thule). Its superb, even tilts back when loaded to allow access to the tailgate. Only thing I had to do was to change the 13pin Euro plug to a 7pin one, unfortunately this means the reverse light wont work on it unless you have a 7s socket too.
It just clamps and locks onto the ball, takes 10sec to fit.
Steve