Niterider v. Superflash. FIGHT
I just got my PB Superflash in and rigged it up to compare to the Niterider taillight I already had. Going to try to make a video at some point, but for now a subjective review: the Niterider's brighter by a decent bit. Of course, for twice the price not including a battery it should be. The difference isn't huge, perhaps 20-30% brighter subjectively, but it is real. Did the test from about 20 feet, at night, standing both directly behind and off at a 30 degreeish angle.
Just wanted to add a datum to the forum, since I've never seen this comparison done directly--perhaps since they're rather different beasts in terms of cost and required other components.