I am a long-time foreign resident of Sofia, so not your typical candidate for a tour. I had a guest coming into to town for one day and the tour was recommended to us by a friend. It was a good suggestion.
The tour starts from convenient location in the center and lasts for roughly two hours. It winds through the streets and past most of the key places of interest downtown. Our guide, Radina, was enthusiastic, knowledgeable and enjoyable to be around. She interacted a lot with the group, which in my case included people from all over the globe.
The great thing about the tour is that it provides a quick foundation of the major sites and their location. Those with time can go back, enter buildings they are interested in and linger. Others can move on. Either way, the tour provides a foundation so you feel like you have a sense of the city and its history.
I think the tour could improve by pointing out some additional attractions. For example, when we visited Alexander Nevsky square, it would have been good to point out that there is a very good icon museum in the basement at Nevksy, and the newly opened crypts below Saint Sofia are a must see.
While I think that there could be some minor tweaks to improve the tour, overall, I think it is great way to learn about Sofia quickly in a fun, group setting.
The tour is provided free of charge. However, be nice and tip your guide if you think they did a good job.