I'd previously bought a "Cabin Max Purple Backpack Flight Approved Carry On Bag", and thought another one would be useful for a family trip. I saw this one and liked the look of the divider in the middle and the fact the zip was arranged suitcase style rather than backpack style. However, it has arrived today and it definitely bigger than the purple one, so much so that I can see it will have to be packed very carefully in order to avoid becoming bigger than the luggage allowance. I measured it with a tape measure, and although the back, more rigid part is the required 55x40, when I measured the other side flat it came in at 59cm wide. Clearly, as it's soft, it can be squashed to meet the appropriate dimensions, but I'd rather not have to worry about that when packing. Note that the internal divider is exactly 40cm long, which shows some lack of attention to design detail - internal dimensions need to be a bit smaller than external dimensions in order to fit neatly! So when the case is squashed to 40cm wide, the internal divider is no longer straight.
If I had time I'd swap it for another of the ones I bought earlier, but I don't think I will have time before we leave.
It's annoying because although I'd seen the reviews on this one and the comments about it being challenging to pack correctly, having seen the purple one I thought the reviewers were being oversensitive - two products from the same company, listed with the same dimensions, should have the same dimensions, shouldn't they? However, the other reviewers are not being oversensitive. Go for the backpack style (also comes in black) instead and you'll have less stresses about overfilling.