Pros
Reusability - Ok, this one is obvious, reusability of the Orbiter and SRBs freed the System of continuous production (which is the case of for eg. Soyuz). So that cut the costs.
Flexibility - The shuttle was designed according to requirements set by the military. The crucial factor in the size and shape of the Shuttle Orbiter was the requirement that it be able to accommodate the largest planned commercial and military satellites, and have over 1,000 mile cross-range recovery range to meet the requirement for classified USAF missions for a once-around abort from a launch to a polar orbit. The ability that differentiates so much from other spacecraft is the ability to retrieve (bulk) cargo from orbit, for ground repairs or for retrieving experiments. It has been used for deploying, repairing and retrieving satellites and the Hubble, performing experiments in space and docking to and building of a space station and crew rotation.
An engineering feat - It was a symbol of the entire nation and a symbol of it's technological supremacy. It was really sexy I must say.. (I'm not American)
Experience - This is more like a side effect, but in my opinion, it is big (right?). Running the same thing for 30 years made NASA know it better with each flight. Only Soyuz could be given the title of the most "experienced" system