Incidentally one of the bags that really impressd me was a racktop bag on a Tailfin rack. Easy to open, easy to close, everything accessible and nothing falling out when it was open; seemed to tick all the boxes (not that I am in the market for one!)
I quite liked the look of them too, but they are very expensive! I'm struggling to find a saddlebag that fits, so have been investigating and was surprised to see that they are also not actually that light. I think something like a tubus fly lightweight rack with a generic zip up dry bag might be a much cheaper alternative (not quite as good, I admit, but might be worth an experiment)
FWIW, I have both. The Tailfin is a whole other thing. The AP20 trunk bag might be heavier than my trusty old Carradice, but it's just... Better. In all sorts of ways. It weighs a bit more because of the frame within the bag that allows it to fix immovably to the Tailfin rack, which weighs almost nothing. Certainly less than even the Tortec Ultralight, which I've got on my fixie. I suppose I could weigh the Tortec and the Carradice for comparison with the Tailfin if anyone is super interested, but I'm really impressed with it. (I have <100mm of seat post showing on my Hurricane, so limited options for luggage.)
Also, the ability to take the Tailfin off in about 10s (assuming you don't have the security screws in place) is a bonus. Means you can swap it between bikes really easily, even if they don't have rack mounts.
The bag also has a side pocket and a side access zip, so if you are organised with your packing, you don't necessarily need to open the whole thing. I find it much easier to pack in an organised way than the Carradice, and obviously the way it mounts is far, far superior to faffing about with 4 velcro straps or a bungee. I can't imagine being able to pack a standard rolltop dry bag the way you can the Tailfin trunk bag. I don't know if you can get dry bags that open on the long side).
Sam