I did the tour last week (Monday) in the evening. Our guide Alex spoke very good English and led us through Sofia’s history and greatest places in a brief and understandable way.
I liked the fact that this organization works with volunteers that have a normal job or study besides this instead of being all architects and historians, so they can offer a more entertaining point of view rather than going too much into (boring) details.
So, in summary, this tour gives you a good overview of the city, with some nice stories told by the guide. When you do it the first day of your stay in Sofia, it is also a nice starting point after which you can decide to visit some things again, but then more profoundly.