Next week I have a 7 hour transit in SFO. Unfortunately it is in the evening (17:45 - 00:55). Do you think it is worth taking the BART downtown? Any suggestions on how to spend the time? Eg somewhere to eat and/or be a tourist at that time of day?
Definitely doable, you could take the BART to Embarcadero, then walk along the Piers toward Fisherman's Wharf to get some late dinner. Then a trolley back toward Embarcadero/Powell Street for the BART back to the airport. Might want to check the timetable for last services though.
Alternatively bayside park might be nice (near the airport) if it's still light, grab a burger from In n Out nearby and get back to the airport a bit earlier and relax with a drink or two in the lounge (if you have access). Probably the no stress option.