When I was researching luggage options for the brompton, I was looking at a carradice saddle bag along with support system very similar to Qr Bagman. however, the largest carradice saddlebag that i could find had a capacity of 24L. the xootr crossbag is basically a large 30L pannier. it is waterproof (to a certain degree) so i can load stuff into a normal backpack and put the entire thing into a the crossbag when i'm commuting to work. but if I want to go on longer rides i can just stuff the crossbag with clothes, food, repair stuff, helmet and other junk. So basically, i have the option of multiple compartments of a backpack, or the non obstructive carrying capacity of brompton rack sack (http://www.cyclingmadeeasy.co.uk/pro...78589_zoom.jpg). Saddle bags stickout more to the sides than the crossbag too. I bought the cross-rack because it was designed to be used with the crossbag. and like i mentioned earlier, i can easily mount it to either of my bikes quickly. But like you said it is pretty ugly and probably isn't built as well as the more expensive saddlebag options.
Here is one on a dahon curve
http://farm3.static.flickr.com/2747/...4af0f74b52.jpg
http://3.bp.blogspot.com/-rEP6WyR4kc...ed+on+rear.JPG