Tom:
Bear in mind all my comments relate to the prototype; I haven't seen the final production version yet, and I know there were some changes made. For example, the prototype didn't have the daisy chain atop the top pouch.
We couldn't take the prototype out for a spin. We just loaded all my wife's gear into it to see how it would fit her and how well it would carry. She loved it – a much better fit than with the Essence, given her short torso length and the Traveler's adjustable shoulder strap attachment system vs. the fixed straps of the Essence. We would have bought it right then and there if I hadn't known that Ron was taking it to the ADZPCTKO as part of his display.
We didn't check the lower right side mesh pocket for water bottle accessibility because we use a Platypus Hoser set-up instead. The hydration port goes through the top right-shoulder corner of the main packbag, not through the top pocket, so its designed to carry the bladder in the main compartment. We didn't try the pocket for a 2 liter bladder fit, but I suspect it would fit OK. You'd have to leave the zipper open just enough to run the hose out (if you're using a hoser system). I can tell you that a Tarp Tent Double Rainbow with poles fit nicely into the long left-side mesh pocket.
Yes, the panel opening is huge and unlike the Essence with its 2 straight, independent #3 YKK zippers and bottom velcro closure, the Traveler sports a single heftier #4.5 YKK inverted U-shaped zipper with double sliders. This is a much more secure closure. There is a storm flap over the zipper as well.
Carter:
Top loaders are far more popular than panel loaders, which is why you see very few manufacturers offering panel loaders. Brian at ULA had a trial balloon thread running here and on other forums, trying to decide whether to add one to his line. Like the SUL 4 oz packs, it is definitely a niche market.
I own both types, and all things being equal, much prefer the panel. You do have to pack them a bit differently than you do a top loader, just like you load an internal frame differently than you do an external frame pack. Each type has its own + and -'s. For example, with a panel loader, you want a small waterproof sheet on which to lay it if the ground is wet or muddy so you don't get your shoulder straps and hip belt dirty.