Definitely the cheapest way to get to Bulgaria direct.
You always get a cooked meal on your flights and free drinks. This is not always the case with some of the more expensive carriers flying direct to Bulgaria.
Unfortunately although their fleet is being modernised and is now mostly Boeings, they still have some quite old aircraft. The comfort levels on their older planes is much lower as things like air conditioning, seats etc. are now outmoded.
On the positive side, if you get one of their newer Boeings (this is a relative term however!) the flight is just a comfortable as the competitors and substantially cheaper.
As someone who flights to Bulgaria on business regularly (although I'm flying in economy class), once you know the routes with the good planes, I often book this airline.
Generally the most competitive routes have the best planes. That means London to Sofia and similar. The worst planes are reserved for flights to secondary airports i.e. regionals.
Where do I start? I flew Edinburgh to Bulgaria on what can only be described as an old russian tin bird! There were cracks everywhere, air con barely worked and the flight was like a sauna. The stewards/esses spoke very little english. Toilet covered in graffity and original toilet seat replaced by a wooden b&q toilet seat that was way too big for the toilet.
On return flight I thought I was going to die, the plane just dropped and the pilot who also did not speak english sounded stoned! STAY AWAY