This is a great, full-size co-op with a wide variety of vegan staples. But when I evaluate a co-op, I look more at the fresh, daily vegan offerings they have and William Street didn't let me down.
They have at least four or five different vegan baked goods daily, all of which attracted numerous people (at least when I was there). Some options included brownies, cakes and cookies. I had a "grasshopper brownie" ($1.45 for a decent-sized chunk) which was moist and rich, though with a slight metallic aftertaste from the mint frosting. I'd still get another, though, as I love the chocolate-mint combination.
The juice bar has OatsCreme softserve, malts, shakes and smoothies, and offers a variety of unusual tea- and coffee-based drinks with the option for soy, rice, almond or hemp milk. I had a 12 oz. apple chai with almond milk ($3.50) that was flavorful, though it could have used more milk for some extra creaminess.
The deli section has some simple tofu sandwich options, including cajun baked tofu, or fried tofu; there were also some cold curry-style tofu and tempeh dishes available by the pound.
Lastly, I just liked the vibe of this place. It was full of hippies, ex-hippies, families, and smart consumers. If I lived in Madison, Williamson Street Co-op would definitely be a major part of my life.