We had a purposefully long layover in Singapore, and this tour was the perfect way to see the city. Our guide, Lynne, was fantastic. Note that while she did provide great information, she did not accompany us to each site, which threw us off at first. However, that worked out better than we had hoped. Because parking is impossible in Singapore, she dropped us off at each location, and we just texted her when we were done; she was there quickly, and we went on to the next location. We hit all of the key tourist sites and then some.
The Changi Airport is like none other in the world. If you have a layover of 4 hours or less, I would spend it exploring the airport. If you have a longer layover (ours was 10 hours), then definitely see Singapore; it's stunning, and doing it with Lynn is the way to go.