You can buy these kits in most motor accessory outlets, quite cheap too, and like most things manufactured, the individual components probably come out of the same oriental production facilities as those fitted to prestige brands, so there shouldn't be too many quality issues. The factory-fitted system on my wife's Rover 25 are still working perfectly 5 years on. I think they're going to be a boon on a high-set estate like the X-Trail, so I bought a kit for mine but have never got round to fitting it in three years! I blame three wet summers!
One tip I did read somewhere is that when fitting the sensors, don't drill holes in the bumper moulding at first. Fit the sensors with electrical tape to the bumper skin. Wire in the system. Make sure it works, and determine the optimum positions for the sensors. This may vary from car to car, and the presence of a fixed tow hitch may affect the centre pair's locations on the rear bumper. When everything's OK, you can do the 'expensive' bit- i.e. drilling holes in your umpteen hundred pound bumper skin!
By the way, the kit I bought included a special drill bit for moulded bumpers.