We stayed in St Petersburg for two nights two weeks ago.
I pre-booked a car transfer from the airport (domestic terminal) to the hotel (HI on Moskovskye Vorota), and back.
It cost 45 USD each way, the driver (same both times) was very nice and could speak excellent english, and was on time. It took 20-25 minutes to drive in both directions. Car was an Open Zafira mini-van IIRC.
I booked it online on
www.saint-petersburg.com, which then books you with Leon Transfer transport company. If I'm returning sometime and I've got luggage again I'll certainly use them again, otherwise without luggage I'll just use the local bus and the metro to reach the city.