The nextmuni predictions are pretty good for buses that are already running on the route. The predictions break down as soon as it has to estimate when the next driver is going to actually start the route.

It's accurate insofar as bus drivers check the time as they drive. This is about as accurate as it is in Toronto, which I consider to have a good public transit system. YMMV

Schedules aren't accurate, but I find neztmuni to be accurate about 90% of the time. It's the only reason non-metro Muni is even usable for me.