Basically the point I want to make is that the planes they use are exactly the same as those used by any of the mainstream commercial airlines, and as they are flying in the EU they have to meet the same maintenance and safety standards as those flown by Sleazyjet, BA and all the others.
Don't worry about the flight it's all part of the experience, and the planes are perfectly safe - just cosmetically challenged
we have had 4 flights with them and they are fine.not generous with drinks but just the same as any other older plane. I have flown on inrernal flights in Russia much much scarier than Balkan. infact the staff were really helpful on saturday when i was struggling with luggage and a broken wrist. ENJOY