I spent a week in Sofia in 1989 just before Communism ended. I was one of only 50 westerners in western Bulgaria at the time and I had two secret police 'guides' who accompanied me everywhere I went. They were a complete surprise as nothing had been mentioned when I booked the Eurobreaks city break at my local travel agent but they met me at the airport, took my passport and never let me out of their sight until they took me back to the airport at the end of the week.
That having been said, they gave me a wonderful time. We had one of the few cars that could obtain fuel and they took me all around blacked out and empty Sofia and the Rila Mountains. Mount Vitosha overlooks the city and the views are fabulous. Borovets ski resort is also nearby and Rilski Monastery lies in the mountains about two hours south of Sofia.
The city centre itself was modelled in the 1950s on central Moscow and it also lies on two levels. Modern Sofia is above ground but you can descend many flights of steps around the city into Roman Serdica which the Communists spent a great effort to excavate and preserve. You walk along Roman streets and into shops, houses etc. It's incredible - like walking around Pompeii but underground. Sofia Zoo lies about 5 miles outside the city. I spent all day there by myself as no one could get fuel, but I'm sure its different now. As with many former Eastern-bloc cities, the architecture of Sofia is stunning and the history is long and rich.
It depends of course northboy on whether you like the resorts or want to experience Bulgaria. The bars and clubs of places like Sunny Beach are the same as Ibiza or anywhere else (why go all the way to Bulgaria for that?). If you want to experience Bulgaria and the Bulgarian culture and people I would recommend Sofia and it's surroundings.