Even those Ortliebs will leak if submerged.
yes and no....
should say here that i'm a bit of an Ortlieb fan. have their panniers and had several of their stuff sacks and rucksacks.
i've got the flight:
i told my mate that it is totally waterproof so he jumped into the lake with it. he dunked it and swan around for 10 mintues and came ashore. the inside of the bag was totally dry.
i'm pretty confident that the roll top bags will hold up, too. there are water carrying systems from Ortlieb using the roll top system.
but how often do you totally submerge your pack?
i have a few complaints with the Flight.
the mesh on the back wasn't taut enough and when the pack was reasonably full you would end up with the plastic thingy pushing into your back. i solved this by undoing the bolts that hold it in place and having a cobbler resew the mesh onto the holder thingy so that when i screwed it back together it was under more tension.
my second niggle is that there are no compression straps. this means that if your pack isn't 70% full anything you have inside shakes about like crazy when descending.
and lastly the bag sits relatively high on your back. when going down something steep the bag interfers with back of my helmet meaning that i can't fully raise my head to see where i'm going.
it's good for commuting, but IMO isn't a proper mtb pack.
an old uni friend of the GF works for them (the GF went for an interview there, but to my dispair declined the job offer - shitty provincial town, though) and she gave me a bag to test whilst guiding in 2010. (incidently, for her diploma work she deisgned the pannier bags for the recumbent bikes.)
Packamnpro2. a much better pack than the Flight and bang on 20ltr. it has polstered pads instead of a mesh on the back which works just as well and side compression straps. it also has a helmet net thingy.
then there is also this
that is similar to the one above it, but has the mesh thingy for the back. don't know it is suffers the same problem as the Flight.
