I had one fitted to my last car and as already mentioned it firms up the car nicely.
Couple of things to watch out for.
The rear visibility is slightly impaired, not much, and I had the rear strengthening bar fitted, comes with the kit, which provides additional bracing and also somewhere to attach 4 point harnesses. Rob Wells told me that this isn't really needed for road use as the bar provides sufficient protection without but if you do have this installed it does reduce the size of squashy bags you can fit on the parcel shelf.
Had my tonneau altered, took an hour, and very happy with everything. Fortunately never tested its crash worthiness.
