I have the tailrider - not the expedition.
Arkel stuff is very very well made. It's been waterproof for me when I've needed it to be. the zippers are stiff, but that's to keep it water resistant.
It's a lot smaller than you are expecting - until you use it then you realize the interior is magically larger than the exterior.
Mine looks new and has been beaten up pretty good.
I guess it's magic.
Expensive, but I guess worth it. Honestly mostly for the looks and things. I sold a set of Nashbar tail bag/panniers when I got it - they were a lot cheaper and really did the same thing. It is worth the money - but is it? The tailrider looks better, has a smaller form factor, is supposedly more aerodynamic, lighter empty... but it's a lot more money.