I did a longish run today and had some time to think about many things, one of which was workout clothes brands. Here are my general impressions of various brands.
Nike (Greek for "victory" which was shouted right before Pheidippides dropped dead after running 26.2 miles -- there's your history lesson for the day!): Overpriced and I don't like to support "the leader." But I LOVE their stuff and they do a great job of marketing themselves as a lifestyle. Plus the "Just Do It!" slogan is wonderful.
Adidas: LOVE the triple stripes down the leg of their shorts and pants, which makes legs look longer and leaner. (I don't think I'll ever get a tattoo -- like 99.9% sure of this -- but if I DID get one, I think it would be some sort of a design down the side of my leg!) Their tops fit great and their prices seem a bit more moderate then Nike.
Under Armor: I like a lot of what they have. They sure seem to run small, and their sizing is a bit erratic (I own a small, medium, and large t-shirt from them, and the medium is the tightest of the bunch!).
Reebok: NASM, my personal trainer certification company, partners with Reebok, so I may be checking them out more in the future out of loyalty.