I've only flown on Bulgaria once (or twice, if you look at it another way, it was a return ticket...) as SLF and it was a perfectly normal experience. LHR-SOF-LHR. We had the minor issue on the return leg after an aborted t/o with engineers called in but in all honesty an entirely trouble-free flight with lovely staff and a healthy trolly as I recall!
Enjoy your flight.
Edited to say no delay to flight either way.
Last edited by ex-XL-in-exile; 29th Dec 2012 at 19:28.