Our January visit to San Francisco included a visit to the park above the Transbay Transit Center. We did walk down through the Center itself while we were there. It was a weekend and therefore pretty quiet. From the park, you can see the glass dome over the center below. We peered in while we were up there prior to heading downstairs. We also saw a pretty interesting cable stayed bridge that brings a roadway in for the buses to arrive and depart.
Inside, the use of the glass domes makes for a pretty airy feeling space. When no one is around, it all feels pretty relaxing. I imagine during rush hour (or what was rush hour before we stopped going to work) it would be a bit less relaxed and rather more urgent!