Hi Richard,
For what it's worth, I'm very pleased with my Ortlieb Bike Packer Plus rear panniers.
It took awhile to learn their intricacies, but I love them. They use a double drawstring for primary closure (be sure to check the example you're buying; several models I tried in the store had sticky drawstrings that were a pain to close). The cap-top lids actually comprise semi-external, very large pockets. I use mine to store my walking shoes on one side and my rain gear on the other, along with my first aid kit, all for ready access. The internal pocket is a large envelope type with a zippered mesh one-third-size compartment. The trick to using them both effectively is to free them from where they are trapped next to the stiffener and lay the pocket(s) atop the load, beneath the drawstring closure. That makes them readily available and accessible for whatever. By folding them under you have, in effect, two envelope pockets and a third that is zippered mesh -- in each pannier. These, with the capped tops, pretty well address the criticism that Ortliebs have no pockets; one just has to find them!
Of course, Ortlieb also offer external accessory pockets that can be attached as you wish. A favorite trick of long-distance tourists is to use the pockets for storage inside the bags, then move them outside as needed to allow for greater load-capacity in the main compartment when needed.
The Plus (cordura) bags are a bit lighter than the Classics (made of truck tarp material), but cannot be wiped clean as easily and I think the cordura is ultimately more susceptible to sun fading.
The Packer (cap-top models) are as waterproof as the Rollers so long as the bike remains upright. They are not waterproof if submersed, 'cos there is still a small opening in the drawstring top through which water could enter if the bag is overturned or submerged. I think the Packers allow faster access, and I like the extensibility allowed by the double-drawstring closure. The quick-release buckles can be let out for carrying oversized loads, as when one picks up large food items like baguettes and melons and such. I always felt the Rollers (roll-top closures) had a lot of extra fabric that wasn't used for anything except to, well, "roll-up", though they surely have their supporters and make for some wonderful bags that are 100% waterproof in all circumstances. I surely considered them, but the Packers won out thanks to the (for me) more versatile tops. The Packers are waterproof for all conventional uses.
In any case, if you go with the Ortliebs, I would suggest a spare stabilizer fin and extra hook set per rear bag. Being larger, they tend to carry heavier loads, and it is so nice to have spares self-stored on the same rail in the event a set is needed to repair crash damage. The extra set of hooks also serves as a slight additional theft deterrent.
Hope this helps; any more questions, feel free to ask.
Best,
Dan.
