In short: working in the startup scene won't be a problem; grocery shopping might be an adventure. ;-)
Consider the similar phrase, "Did you eat good pie", versus "Did you eat pie well". In the pie case the former is correct because you're referring to the quality of the pie. In the case at hand, we're talking about the quality of the speaking.
http://www.deutsche-startups.de/2007/11/29/berlin-ist-die-st...
Plus, as has been mentioned here several times, you can't just up and go to SFO without the appropriate paperwork. You could spend three months there, but that's really more like an extended holiday than really moving.
As for English in Berlin, it's definitely doable. Socially I really find it worthwhile to put effort into learning German since otherwise you're often stuck being the odd-man-out when going out in a group or hanging out with other foreigners.
The Valley maybe, but not San Francisco and the Bay Area generally. San Francisco has an extremely mature (though not in a classic sense) arts and music scene.
Right now Berlin's one of, if not the, place to be for arts and culture in the west. Which was kind of the point of the analogy -- the Valley / San Francisco is the epicenter for the startup world. Berlin is the epicenter for new music, art, etc. (And this isn't just coming from a biased Berliner; I'm an American that ended found his way here. :-) )