I can't praise this service enough. The price we were quoted by Welcometaxi (just under £25 for the journey during a Friday evening rush-hour - Thanks Elana for a sterling job getting through the maze of stationery traffic!) for a return from a St. Petersburg central hotel to the airport was more than reasonable and a third of what was quoted to us by the hotel, had we booked through them. The driver came into our hotel, had a sign with our names on and made the whole return experience a joy.
On arrival in St. P, We had intended to get a bus from the airport to the hotel but got on the wrong bus and narrowly missed a fine thanks to a helpful English speaker who saved us. Deciding we had to take a taxi to the hotel from the plethora of taxi drivers outside the airport, the cost was nearly £45! Wish I'd booked through Welcometaxi before I arrived!
Don't try and save money - book through this company. Russia is not easy to get around if you don't know the area, and the cost to your hotel and back is minimal for the piece of mind. It is seriously worth it. Very highly recommended!