Coca Cafe is amazing! I moved away from Pittsburgh recently and I miss it every Sunday... Their food is great, the waitstaff is great. Yes, they are crowded on the weekend, but so is every other popular brunch spot in the city. Tip: call ahead before you leave your house to put your name on the list. Then, as someone else mentioned, you're free to wander Butler Street until they call your cell!
I recommend the challah french toast, goat cheese french toast, sundried tomato omelette, and all of the specials. Yesterday I had cornmeal waffles with pomegranate reduction and pineapple curd... yum!