I have only been there skiing so not sure much help seeing as you are going on a summer holiday. I really liked it, being a ski resort it was probably more expensive than other parts of Bulgaria but still a lot cheaper than UK. I saw a bit of Sofia on the way back too which was good to see a little. We ate a lot of meat though!
Have you been to other countries in eastern Europe or first visit? I quite enjoy visiting eastern European countries in general and have been to Latvia, Estonia, Poland, Czech Rep, Slovakia, Slovenia, Serbia, Croatia, Hungary, Bulgaria, Romania, Georgia (geographically Asia I suppose).