While the weather may be basically the same in San Francisco all year, I much rather visit during the winter. Minus 5 degrees in Chicago means 60 degrees in San Francisco is awesome, but 85 in Chicago means it's too cold in the Bay Area.
My first trip to San Francisco was in college during our West Coast volleyball trip, but you don't really get to see much when you are traveling with 12 other guys in passenger vans.
It took five years for me to return to the city and get a more in depth tour - not that my girlfriend at the time (now my wife) was the greatest tour guide. Me: "What's that?" Her: "I have no idea."
However, food and restaurants, that is her strong suit. We enjoyed a nice lunch at Water Bar with their oyster happy hour. Dinner at Range was a good meal.