I think Under Armour is effective at reducing weight from sweat, particularly when running. A cotton shirt does seem to hold more of it. Doesn't have to be that name brand, but I guess they developed the material?
Other than that, it's just another stupid-ass label that people buy. I really hate people who have crap like Under Armour caps.
But, hey, there's a lot of stupid people out there that deserved to be ripped off.