I'm moving to San Francisco and the place I'm looking at is at shotwell st. and 21st in The Mission. Safe?

The person said the block is nice, but the rest of the neighborhood is a little sketchy. I live by Columbia University in NYC right now which isn't too bad, but is close to Harlem and has some crackheads and crazys, etc. The price of the place in SF is only 660 and looks like a sweet apartment, close to a lot of things. However, I'm trying to live in an area where I'll feel safe at night. Any advice?