The storage box lifts out very easily and can be put in the boot - it is not bolted to anything but there are two holes in the rear which fit over the carpet/mat retaining pegs. I find it provides useful additional storage inside the car (mine's full of CDs) and keeps items away from prying eyes. You can ony take one adult passenger in the back when it is installed and I would not recommend that even a smallish child sit on the same side as the under-seat storage box (I reckon it could cause problems for someone sitting on that side of the car if it was involved in a hefty prang - if their legs are long enough for their feet to rest on the top of the storage box). It also has to be removed if you want to fold the rear seats fully.
Even from vagparts.com it's a fairly expensive accessory so, if it comes with the car, take it as a bonus but it won't add anything to the value of the car.
P.S. It does not rattle and, being lined with carpet, nor should what you put in it.