I'm the other one, except for the fact parts are made of unobtanium. They are lighter, and take less power away from the wheels (like a 904 compared to a 727). I wouldn't use a 7 1/4 behind a stick trans, and submitting it to shock loads, but they hold up pretty good with a built slant or mild 273/318 and a auto trans.
PS: the sure grip spiders are stronger because there are 4 spider gears, unlike the open rears which only have two spiders.