I saw two performances at the Mariinseky and enjoyed them both. I really enjoyed hearing the 1812 Overture played in Russia! The ballet was also wonderful.
A tip to travelers: If you buy tickets through your hotel or a tourist place in the city you will get completely ripped off on the tickets. I was a student so I was able to get student tickets at the box office for $10. I watched a couple in front of me buy full price tickets for $40. They were foreigners but spoke enough Russian to get tickets. At the performance we sat next to some other Americans who had paid $100 per ticket through their tour group! So, beware! If you can get a local to buy you the tickets, do it. They have special rates (or something).