Well, from personal expirience, Bulgarian and Serbian people like eachother. I was in Bulgaria, we had few Bulgarians on our Faculty, etc. From what I saw Bulagarians love Serbian culture, our music (bad and good ), our movies, poetry, our TV etc. Serbs, on other hand are really disapointed and disilusioned in Bulgarian Government and its servile mentality towards US,NATO and EU, but do not blame Bulgarians for that, and we mostly like them, but must admit that we are bit arogant towards them.
Concerning language, I think that Bulgarians understand Serbian better than we understand Bulgarian.