I can't comment on the other boxes, I was limited to what I could find in the city.
My first Givi was one actually designed for a scooter, I bought it mainly because it wasn't very big (30-odd litres), and hence not that expensive. I bought it after getting extremely pissed off with carrying a Ventura bag to work on the train every day (and no, I don't trust convertibles either

).
The box fitted one full-face helmet for a boofhead perfectly, but had issues with my Targus Laptop Backpack
IF it was more than around 70% full. Less than 70% full and you could fold the top down and all was right with the world. Over 70% and the bag had to go on your back.
The new box is bigger, and is a different shape - it's actually a dual-purpose box and can be used for either a top or side case. It's meant to take two full-face helmets, but I gather they must be only those that fit Barbie and Ken or Hobbits. My boofhead helmet fits in exactly the middle and only in ONE position, otherwise the lid won't shut. On the upside my bag now fits however full it is (as long as the straps don't get caught in either the hinge or the front). I can also fit other crap in there like two pairs of gloves and a jumper.
Good points:
* Solid, locks to the bike extremely well and shuts like a clam. Good Italian engineering in both the box and the rack.
* Completely and utterly
100% waterproof. And I mean 100%, you wouldn't believe some of the rain we (used to) get out here. The bottom of the case has a recessed flat-faced rubber o-ring that goes around the entire circumference of the box. The top of the box pushes into this and is locked in place by the front clasp.
* Cheap replacement keys.
* The plain matt black doesn't look
too wanky, especially if you DON'T deploy the high level brake lights in the reflector.
* They seem to be designed with the balance of the bike in mind and don't affect the handling at all.
* They are really easy to get on and off and the exposed plate doesn't look to bad (unlike a Gearsack rack, which is really ugly).
Bad Points:
* They are
really expensive, and paying for the rack and box is like passing a kidney stone.
* The instructions are bloody woeful, too small and too much detail in too small an area. Having said that, that's not just a criticism that Givi is guilty of, the Honda flyscreen ones are crap as well.
* In their infinite wisdon Givi re-invents their rack system every now and again, negating your attempt at recycling your old box and forcing you to buy a new one, especially if you have a fairly recent design bike

.
* Anything (like small change) you carry in your bag in the big box ends up spread everywhere inside your bag.
* Don't carry Chinese takeaway in a Givi box UNLESS you like Givi box-flavoured takeaway (I don't waste a Seafood Combiniation unless it growls at me when I open the container).
* Supposedly they have a 120km/h speed limit. I gather this has a bit of flexibility as I know they don't fly off above this

.
Would I buy another one? Shit yeah, the good outweighs the bad in my book

...
...did I mention they were 100% waterproof?
HTH
Mick