Has saved me twice.
The first day I arrived, after the mix-up with the butcher, a bespectacled older man carrying a small dog opened both downstairs doors and helped me find the buzzer.
The other night, after struggling with the monster door for a full ten minutes (the key turned the deadbolts, but not the latch bolt) I was about to give up hope. Then Gilberto appeared with his little dog, I explained my problems, and he disappeared to deposit the dog in his apartment, reappearing with some WD-40. The lock had finally given way by the time he returned, but he oiled everything up nicely for me.
It's nice to have a guardian angel in the building.