Top of the park is a very cute little place, emphasis on the little. Every time I have been there I have left quickly because it's always so packed. It also tends to attract and older crowd (possibly because of the steep prices?).
The pianists are all very entertaining and there are frequently additional instrumentalists that make appearances, which also makes the place smaller.
Seeing the city and bay from Top of the Park is a must do both day and night, but I wouldn't spend an entire night there.