Unlike staehpj1, I have little positive to say about Nashbar panniers. Perhaps they have a better model than the cheap one I got. The only good thing about it is that it converts to a pretty nice knapsack. But it lacks stiffness, so it would wobble, and get caught in the spokes. And then there isn't enough springiness in the elastic that is supposed to hold them down, so they fell off my bike. My laptop still bears the wounds.
Previously I had some 80s-era Cannondales, that finally wore out. I probably paid over $120 for each of them in 1983. But they lasted about 25 years, including two tours, and lots of commutes. The Nashbar lasted a bit over one year.
As to the question of Arkel, they are very well made. The guarantee enumerates everything that ever goes wrong on panniers, and covers them. The attachment takes some adjustment to fit the racks we have (on singles and a tandem), but once adjusted, they are quite easy on and off, and quite secure once on. The lower priced ones are just a touch more fiddly to put on, but still secure. I fully expect our Arkels to last well in excess of 25 years, barring a catastrophic accident. At our age, that means I don't expect to ever need to replace them. Amortized over that many years/miles they are quite inexpensive.
But the majority of Arkel's line is designed for the sort of person who doesn't like having to rummage in one cavernous bag for the thing that seems to have made its way to the bottom. That is, it has pockets. And if you're the sort that prefers to create your own organization within a large space, that's not adding value for you, only cost. We are pockets people. And somewhat suspicious of a bag that is truly waterproof for what happens when you put something wet in? Does it every dry?
So yes, Arkels are very well made. So, I'm told, are Ortliebs. In my opinion the extra money is worth it for the longevity.