For me, the best part of visiting St. Petersburg were the famed White Nights. Admittedly, since we visited in early May, we didn't get to experience a full twenty-four hour's daylight, but we were still treated to three or four more hours more than one would get in most of the US. This may not seem like much, but it's more than enough to induce considerable disorientation: I walked out of a resturaunt at ten o'clock, but as bright as it was outside I could have sworn it was only six or seven.
While the sun finally does set around 10:30, the following twilight continues on for several hours more. The contrast between the little remaining glow from the sky, the recently-lit streetlamps, and the still-bright horizon gives the city an eery appearance that's excellent to stroll around in.
Most of the city's landmarks become more attractive towards dusk, but not all. In particular, the TV tower is illuminated at night by a number of small, rapidly sparkling lights, which cause the tower to stick out garishly above the otherwise low-set, not sparkling skyline of Vasilevskii Island.