Late to the party but I visited Saint Petersburg during summer while staying in Finland. Wow, what a great city it is I agree with madhusudana! Definitely, travel during summer, and make sure you're in there for the festival of the Scarlett Sails (or something like that). The fireworks are friggin amazing, Dubai-level to say the least just have a look at the picture:
Apart from that, there are a million other things to see: museums, theatres, ballet, and the nightlife is great. I felt safer than at home for the most part (and I'm Swiss, lol!).
So yeah, 10/10, must visit!