They get a mention in Shawn B's suspension thread. I've also talked about them (sort of) in
another rear suspension thread which you should read. It will help you understand the rear suspension a lot better if you don't have a solid grasp on it already.
I've heard of a few racers using something like it (adjustable length parallel links) usually with spherical bearings just because they're race cars.
They give you camber adjustment in the rear (independent of toe adjustment) to either compensate for a shorter ride height, or you know, take advantage of more (or less) camber. They also have stiffer bushings than stock so than can be a plus or minus (might cause slight increase in rear suspension bind).
All in all I think they are an okay piece but most of us don't need the adjustment, and those who do would probably want spherical bearings. They are neat because they are a direct drop-in, and they may be the right thing for a few people...
They are possibly cheaper than OEM replacement parallel links, so that's a plus...