See this thread http://www.flyertalk.com/forum/europ...-bulgaria.html
As a local (born there, but not live there any more), I'd say it's a nice city. It's not a major tourist destination, but if you want nice (and cheap) places to eat or drink or clubs (dance or live music), it's a very good choice. There is a share of historic sites as well, but it certainly is not Rome or something like that. Plovdiv has a historic center, which Sofia lacks. It's about 2 hours by bus.