I would rate this trip as a must-see in Sydney.
The weather was fantastic the day we took the Big Bus. We enjoyed many views from the second floor, riding the bus around the entire city center. We saw many cityscapes and cultural moments that are impossible to experience by public transport. The entire tour had a Chinese-speaking guide, with explanations at every point in the city in Chinese, allowing us to learn a lot about Sydney that we couldn't have explored on our own.
The red line in the city center was excellent, but the blue line was slightly less impressive. The coastal area wasn't very extensive, and the wind was incredibly strong that day; sitting on the second floor, we were practically blown around like idiots, and there weren't many particularly noteworthy views.