Pierre, I have a GIVI Maxia E52 (52 litre), and the reasons I chose it are:
It takes 2 full-face helmets (Yamaha takes 1 only), and we often travel 2-up
It was less expensive
It is an easy clip-off, clip-on when I ride solo and want no bags, no top box
I had an adapter plate made that just fits in place of the GIVI baseplate, so I can fit various sizes of soft bags if I want to
Zea has really gotten familiar with the GIVI handholds (not that she holds all that often) and likes the position, but I suspect that this could be un-learned very quickly.
I know some people have lost a rubber spacer? with the original Yamaha trunk, so if you do decide to go that way, pay attention to that item.
Hope this helps.