Sibir (like Aeroflot) have one problem - quality of service may vary between particular flights. On one flight you may get (with Sibir) a fresh Tu154M (say 6-10 years old) with renewed interior and excellent cabin crew which will make your trip better you can expect with even with western high quality carriers. On the next day you may face old doggy dirty Tu154B or old 154M with 20 years old seats and carpets and rude cabin crew which will remind you a domectic sector in Dark Africa.
Sibir is now 2nd biggest Russian airline, and no 1 in terms of number of passengers carried on domestic flights. Safety record is nothing to worry about. Heard from some friends in the States that their corporate travel policy (which is also connected to insurance) is now allowing to travel on Sibir domestic flights without special authorisation, while few years ago it was Aeroflot and Transaero only.