Trams, Trolley buses, Buses and Underground form a fairly comprehensive public transport system in Sofia spreading all the way into the suburbs. The extent to which you decide to use the system depends on where you need to get to, your personal preferences and your navigation skills. Of all, the Underground is probably the most user friendly, modern and clean.
Sofia's other public transport  systems are also getting better and new trams, trolley buses and buses are already running on some of the routes and there are also some routes across the city on which night buses operate.
Plenty of useful info on the website www.sofiatraffic.bg and the moovit app is very helpful and pretty accurate in navigating you from A to B.