Yoga is a state of mind and in my belief what you wear to class does matter. I often wear the same pair of black cut-off sweat pants, sports-bra and black v-neck, and though it's the most comfortable thing I can find to wear to practice, it really doesn't do much for my mood, wearing all black is rarely uplifting.
So I think, very soon, I will try my best to buy some Yoga wear... at Hard Tail Forever, who has a great Yoga wear line.