Sheesh, people have a pretty low tolerance to skeez here! I'll admit SF has its seriously sketchy parts, but they're primarily sketchy if
you are sketchy. Do you deal bunk drugs, owe drug dealers scads of money, or participate in ill-conceiled drive-by shootings in Hunter's Point? If you answered no to the above, you have *very* little to worry about.
That's not to say folks don't get jumped on occasion in the t-loin, but I'd say it's no worse than the troubled parts of NYC or LA. I think the main thing that makes it a little hard to take is that there is no bifurcation of the bad and good parts of town. Like folks are saying, there are hard streets and boutiques within half a block of one another. Just gotta stay a little cognizant, or, really, DON'T. I have lived in and around SF for years and have never had a real issue with anybody.
SF has SO much going on culturewise, funwise, foodwise, artwise, musicwise, etcwise that it's pretty hard to stay focussed on ****-covered hunks of newspaper in the gutter.
That said, for a city with a sky-high cost of living, SF is not terribly clean and not always well-kept. When I take trips to Portland or Seattle I almost feel like I'm visiting some kind of model city that nobody lives in yet. Where's the trash? Where's the puke?
