I've lived here for almost 20 years. BART into downtown from SFO is super-easy, unless your plane arrives late at night since BART stops running around 1130-ish.

Let's see...you'll find more than a few characters at any BART station from 24th St. Mission all the way along Market St to the Embarcadero. If you're staying in Union Square you'll probably want the Powell St. BART stop, and that'll put you right on Market St.

Honestly, if you're not comfortable with 'unsavory' people you probably don't want to come here...Market St., the Tenderloin and Union Square are full of homeless folks because the weather is mild and the city doles out plenty of services to the homeless. I grew up in NYC and we certainly have more homeless people per capita than NYC (at least it certainly seems that way.)

That said, I've never once been hassled beyond the usual "Got a dollar?"

When you take BART out to OAK you might see a few people getting on at the West Oakland and Fruitvale stops, who clearly have done some hard time in CA's state prison system. Fruitvale is where Oscar Grant was shot by a BART cop (you may have heard of the movie.) Perfectly safe during the day, though I would not walk around the Fruitvale neighborhood or West Oakland at night.

That said, SF is a pretty safe city. The worst thing that will happen to you in Union Square is that a homeless guy might start screaming at you for no apparent reason. Ignore him, and enjoy:

• Golden Gate Park...you can spend a couple days here and not see everything
• Golden Gate Bridge
• The restaurants! Oh, the restaurants.
• The awesome neighborhoods. Hayes Valley (fantastic restaurants and wine bars), Noe Valley, Cole Valley (hmm we seem to have a lot of Valleys), South Park, the Inner Richmond and Inner Sunset, the Mission District (world's best burritos)...Jackson Square, the Fillmore, Nob Hill...the list goes on and on.
•*Notice I left out Fisherman's Wharf. No locals ever go there. Ever.
• Escaping to gorgeous Sausalito, Tiburon, Angel Island, and Marin County
• Ocean Beach (bring warm clothes!)
• The Embarcadero
• Wine tasting in Sonoma, the Dry Creek Valley, or the Alexander Valley (note I left out Napa, for good reason. It's an overpriced zoo. The real wine fans go to the Dry Creek or Alexander Valleys.)

Enjoy!