I got the Ortleib Bike Packers Plus (the non-roll-top) about a week ago and I freaking love them.
I was surprised to find they actually have a pocket on the inside where my laptop fits snugly.
As far as less watertight, they have two cinch cords on the top and then a watertight flap that goes down over it. Seems fine, not sure what the disadvantage would be.

To be fair, I haven't actually ridden them in the monsoon...yet. They also hold a ton of stuff (lunchbox, clothes, laptop auxilliaries, laptop all fit in one pannier) and the attachment system feels completely bomb-proof.

Granted I haven't tested them over more than a week or two but they beat the heck out of my old milk crate. ;-)

Nobody, et al.
Acoustic Music for an Electronic World
http://www.nobodyetal.com