Great food, great beer... Definitely a must if you're new to/visiting Pittsburgh. I constantly recommend Fat Heads to friends that are in town visiting looking for recommendations. I think Fat Heads is what Pittsburgh's about! Great food (and lots of it), great beer, and a great atmosphere.
I usually get their veggie burger with fries, which is delicious. I would come for just their fries, in all honesty. SO GOOD.
Their beer is indisputably good. You can find it at a lot of other restaurants in the city as well. They all have fun names, which adds some nice personality.
They have a small outdoor seating area, which is nice in the summer.
If you do plan on going here - get there early. They often have a long wait - but it's always worth it.