The free Sofia tours are as much about understanding the top few attractions of Sofia as it is about exploring the area slowly and with a local perspective. We did the tour in the end of July, and I totally recommend it. The tour is in excellent English (and on the we did it, the assembled group was so large, that we needed 2 guides). We were from over 5 countries and the guide Christina (I HIGHLY recommend her) did a wonderful smiling job of getting to know us, sheparding us and pointing out little quirky details. She was happy to both answer all kinds of questions and admit her extent of knowledge as required.
Anyone visiting Sofia, please do this on Day1. It gives you a great idea of what to expect further.
Since it is free, no complimentary stuff. So, make sure you carry your own bottle of water (esp in summer, even for an Indian!) And do contribute so future travellers can continue to enjoy the tours.