I've been to the History Center on several visits both to see actual exhibits or to attend events that are hosted at the Center. The building itself is historical by its own merit and they've done a wonderful job at restoring it and adapting it to serve such an awesome set of functions: museum, exhibit space, event space, etc.
This is one of those place you have to see to truly appreciate, all of the exhibits I've had the pleasure of seeing have been high-quality. And their standing ones such as Pittsburgh Sports, modern marvels, and memorabilia are ones that you will never get bored with and learn something new every time.
There's is so much to Pittsburgh and its history...so this place is truly special.