I find all boxes, panniers and cases plain ugly. The single ones sticking way out high in the rear look stupid and if full affect the handling, not to mention they stress the subframe. The ones on both sides make a bike look like a pregnant cow and you have to be careful not to knock over pedestrians on the sidewalk.

I've never understood why people would need these huge suitcases when they travel.
What's in there? Can't all be clothes, they would last for a month.
Okay, if you travel two up and have to accommodate your wife's beauty utensils, hair curlers, hair dryer and an ironing board, you need big boxes, but if you're traveling by yourself all you need is some underwear, socks and T-shirts for a week. There are laundry mats all over the place, I rather hang out there for an hour or two than drag those heavy boxes around with me and look like a U-haul truck on two wheels.
I've traveled for weeks that way, all I need is some change of clothes, a razor and a toothbrush. I had a rack welded up and tie a small box with bungee cords right behind me so the weight is distributed better and there's less drag.
Check out the pics how my bike looks with the box on.
Here's a video that shows what may happen when you turn your bike into a station wagon and forget that it's now as wide as a car: