We went with family to see it as a tourist attraction, and it absolutely delivers on the food. Basically the place is a food court featuring some of the best chefs in Houston. We had food from Soy Pinoy (James Beard nominated chefs), ChopnBlok, Golfstrommen (one of Alison King's favorites), Johnny Good Burger, Thai Kun and Rollin Phatties. All of it amazingly good and worth the price.
During the day the food court is the real attraction. The rooftop garden is nice but was warm even on a pretty nice spring day. The view of the city from there is amazing. There are a lot of small vendors selling jewelry and clothing, but not super special. You come for the food.The prices are high, but I would beg to differ with those who say they are outrageous. An average entree around the food court is $20+, but that's not far out of line with a good food truck.
I found this to be a really accessible way to get top-notch cooking. We would go to a fancy restaurant and order from a menu maybe once a year, if that. At POST you can walk up to a counter and order something made by some of the most talented chefs in America. And you can sample from several of them at once, rather than committing to one place. It's a work in progress and I think it will get better as they finish the vendor spaces. Parking is indeed an issue, it's a pretty small lot and kind of expensive. The website recognizes this and lists many options for parking nearby. Not perfect but absolutely worth the experience.