eu01: If I recall well, EZY used to fly to TLL from SXF but decided to discontinue the route. As the amount of passengers from Russia to Western Europe keeps growing, EZY should perhaps reconsider starting the route again. Both Estonia and Germany belong to the Schengen zone, the same visa is valid everywhere.
Besides, Ryanair will soon commence the routes to/from Tallinn and, as you certainly know, already flies to Lappeenranta.
Yes, unfortunately U2 have commenced TLL-SXF and all routes out of RIX at the end of 2008 (start of recession which was rather hard for Baltic states).
I flight it back in August, 2008 and plane was almost full with lots of Russian passengers on board.
TLL is very nice starting point for St. Petersburg passengers, as there is cheap (20 eur) and fast (about 7 hours) coach line Lux Express, operating up to 9 rides/per day between St. Petersburg and Tallinn.
Unfortunately there is no good enough coach connection between St. Petersburg and Lappeenranta at the moment.