Depending on where you're at in Oakland and where you're going in the city, it can be anywhere from 10-45+ min. Not bad if you're in West Oakland or downtown Oakland; much more of a trek if you're in Berkeley or trying to get deeper into the city. For example, if you lived in downtown Oakland, it'd take you about an hour/1:15 to get to the Waller St ledge spot near Golden Gate Park. So not the greatest, but doable. The big bummer about BART and the East Bay is that the last train to get across the bay is at 12:30-1am-ish, making it way more difficult to stick around for a show or a bar or whatever. There's some late-night bus that runs across the bridge a few times while BART's down for the night, but I know nothing about it and my gf says it's super sketch. Of course, if you have a car, that's not a problem...though parking and traffic and whatnot are.
From what I hear, Oakland has a pretty rad skate scene of its own. Also, living in the city doesn't have to be as crazy expensive as people make it out to be. Yes, rent's higher than a lot of other cities, but if you lurk Craigslist and are persistent, you can get into an existing roommate situation for an "affordable" price. Finding an empty apt for you and your would-be roommate is just not going to happen; you're competing with the rich tech asshats who will pay extra rent or pay a year's rent up front or other bullshit like that.
But distance from your work can't be helped, I suppose. Really, Oakland's rad. If I worked on that side of the bay, I'd definitely consider living over there. A number of my good friends do, and they love it.