I have never been, but they say that St Petersburg is more beautiful than Paris.
Well, I've never been to Paris so I can't say, but StP. is definitely worth visiting. I've never seen anything compared to those palaces and jewellery.
The most popular and grandiose palace (aside from the Hermitage) is probably Peterhof (Petergof) about half an hour by car away.
Visiting Pushkin's flat and the Jusupovkij Palace (were Rasputin was murdered) on the Fontanka is of course also a must.
Personally I also liked the Michailovskij Dvorez (Palace) very much. Honestly I was a bit disappointed by the amber room. It's much greater on the pictures.
Mr. Beaverhausen and I were in St Petersburg 2 years ago. So I’m going to weigh in here and say that they are both very beautiful, but I give the edge to Paris. St Petersburg looks like a jewel box with its rebuilt city center, but it’s all very new and looked more Disney to me than Russian. I loved the Amber Room and The Hermitage, but I love the history and ancient architecture of Paris more, and the fact that, in comparison, tourists can more safely walk around without getting mugged. Also, pictures and T shirts of Putin were everywhere in St Petersburg.