I'm referring to getting around
within the city itself via transit.
All of MUNI's light rail lines are gigantic perpetual clusterfuck machines. Their buses are no better.
Getting to/from downtown and to the Outer Sunset easily takes an hour on severely over-capacity rail cars - a distance of a mere 6 miles. At rush hour the cars are literally packed - these are the most crowded transit vehicles I've ever been on on this continent, NYC included.
Their on-time performance is atrocious, frequently late by over 100% of their frequency (e.g., an every-15 schedule routinely results in trains late by more than 15 min).
So not only are the vehicles slow and infrequent, but they routinely fail to show up when they're supposed to. Every major study into public transit use will tell you that schedule reliability is critical to establishing a transportation network - without the ability to reliably make transfers people will simply stop using the system. The last I saw of it, MUNI vehicles were on time only 57% of the time (lateness is defined as being within 4 minutes of schedule, which in any large city transit is a margin as wide as a football field).
Large swathes of the city are entirely not served by anything even pretending to be mass transit - Pac Heights, Inner/Outer Richmond, the Marina, Russian Hill, are all big neighborhoods that are completely disconnected from any form of mass transit, BART, MUNI, or otherwise.
Buses are even worse off than light rail - there have been many efforts to improve the bus system to little effect. Efforts to make commutes substantially shorter (by taking out extremely closely spaced stops) have been repeatedly rejected by members of the public, who will proceed to complain about MUNI anyways.
Literally the only neighborhoods that have effective mass transit are the ones lucky enough to be connected to BART (read: Mission, Bernal, Balboa Park). These are the only neighborhoods that have access to a transportation system that is frequently on-time and actually reasonably fast. The BART has a slew of its own problems, but it's about 10x better than MUNI for getting people around.
SF isn't the worst transit system I've ever had to use regularly (Seattle takes the cake there, just barely), but it's severely under-developed and under-funded for a city of San Francisco's stature and ambitions (i.e., its very open goal of creating a car-free/car-reduced city). I dare say that if you do not own a car in San Francisco you will spend a significant sum on taxis, or Uber, or SideCar, or any of the other services that have been born from City Hall's transportation failure. That or you bike a lot.