I haven't looked at the Nelson-Rigg stuff so can't really say about quality of construction. It looks a lot like the Humpback tail/tank bags I have in both size and shape and they are very functional.
I CAN say that the Oxford Humpback stuff is way nice and well constructed, but it's also almost 3 times that $50 price.
I'm sure it is pretty good and at 50 bucks what the heck.
The tail pack is slightly less convenient than a tank bag and I actually really like the tank bag on there. A little wind block and a place to lay down on long hauls. I does not bother me unless I'm REALLY getting aggressive and I shouldn't be doin' that on the street anywhooo...