When I traded in my Bandit 12 for the GS I kept the Givi box and mounting plate, even though the GS came with full vario luggage. I used the BMW top-box on it's own for a while, commuting and to carry the camera at weekends, and it was OK. Annoying that I couldn't get my helmet in there, even when expanded, but the fact that it was compact, neat, and as waterproof as the Givi meant I used it for most of the winter.
I then got fed up carrying my laptop in the box and my gym stuff in a rucksack,
and not being able to stash my helmet, so I bought the GS fitting kit for my Givi and was happy again straight away.
Last weekend I rode to Nancy, and because I wasn't going to be stopping for sightseeing on the way (helmet park a bonus; on the Bandit my Givi was used solely for this purpose on longer trips) and because I was also taking the Vario panniers I decided to revert back to the Vario box as well.
So in summary, I think the BMW box on it's own is too small to be of any real use. I think I've already said that you can't put your lid in it, even though the ability to operate it without using the key is a bonus over the Givi in my opinion. But if you use panniers as well then you don't really need the extra capacity that a Givi Maxia would give you, so the Vario box is the way forward.
Just my tuppence worth.
Mup.