I used a Groupon offer for my fiance and I to take the bus tour. It was a rainy day and still in the off season for tourism, so for the first portion of our tour, we were the only ones on the bus. This made for some unique interaction and story telling that would not normally have been a part of the tour.
The tour guide and bus driver were both extremely friendly and although the tour itself was fairly scripted, they answered questions we had and tried to make even the more "boring" facts interesting and fun.
Because of the rain, we stayed on the bus for the duration of the tour rather than choosing to "hop off" at any of the stops. It took about 90 minutes which was enough time to take in sights and get a great overview without being overwhelming. It is a great idea for someone new to Pittsburgh or visitors trying to decide what they want to see and do during their time in the city.
Choose a day when it is sunny so you can enjoy the better views from the upper level.