I have a different take on North Beach. I would say that though crime in North Beach is not as bad as say the Mission or the Tenderloin, it is still relatively high for San Francisco, whose overall crime rate is slightly above the national average.
So I looked on this crime tracking website www.crimemapping.com and looked within half mile radius of the corner of Columbus and Union, roughly at the heart of North Beach. Now, although there have not been any murders there in the last six months, here is what there has been:
163 Assaults 114 Burglaries 34 Roberries 29 Sexual Conduct Calls (mostly indecent exposure, but some prostitution busts and a handful of sexual assaults)
I would say that though you probably won't get killed living there, you do want to watch on the side streets if you are going to live there in one of those cute 3 story Victorians. My friends and I lived there one summer and one of them did get mugged onw night just off Columbus on the walk back to her apartment.
That said, it is still a great place to live for a year or two--especially while you are in your 20's.