I've identified the three of the best cities for young professionals in California, having lived in nearly 20 different California cities over the years. But let's start with the one city that people always think of first: San Francisco.
The biggest problem with living San Francisco is the high rents. There's an old joke that to live in San Francisco, you have to be rich enough to afford it - or young enough not to care. I've had the wisdom of that remark confirmed by nearly every person I've talked to who ever lived in San Francisco. If you have a high-paying job, you can find an apartment with a terrific view and a pretty good location. But failing that, you'll have to look hard around for a good deal, settle for a "roommate situation," or resign yourself from living further away from all the action.
Here's why. Unlike most cities, San Francisco is on a peninsula that's surrounded by water, so there's no way to add any more real estate. (In the 1930s, the city of San Francisco even constructed a man-made island in the Bay.) But what's interesting is that a few military facilities are being converted into brand new civilian housing, including developments at Hunters Point and that man-made island, Treasure Island.
Fortunately, the city has a pretty good mass transit system, so you'll still be able to get to where the night life is. And San Francisco is famous for its night life, intertwined with the historic 1960s youth movement that made Haight Street so famous. (There's still lots of good places to catch live music.) In addition, San Francisco is famous for its great restaurants, including lots of great local restaurants where you can get some great meals and authentic international foods at very cheap prices. And San Francisco has a lesser known attraction for young people: there's several great movie theatres, as well as many good film festivals throughout the year - so you'll always have someplace to take a day.
After living in San Francisco, San Jose will seem a little boring by comparison. It's a "newer" city, growing and springing to prominence after an enormous influx of money during the dotcom boom of the 1990s. San Jose acquired their own hockey team, and they've got a very unique museum (devoted to - what else? - technology!) There's lots of new living spaces, and lots of restaurants built to cater to them. The town is practically designed to cater to professional workers, and though it's lacking some of the history and charm of San Francisco, it's also lacking some of the urban congestion and other big city problems.
Finally, there's Los Angeles, which is the biggest city in California, though it's really a cluster of smaller cities. You'll want to choose the right neighborhood if you're looking for interesting nightlife. (Try looking in Long Beach or North Hollywood.) Los Angeles is also the second-biggest city in America, so it's got just about everything you're looking for - somewhere. But it's also a little like living in New York City. You've got to adapt your life to the city's unique challenges - which in the case of Los Angeles means the threat of a traffic jam, at nearly any hour of the day.
I think it's probably easier to grow up in Los Angeles than it is to relocate there as an adult, because there's a real "culture shock" that happens. Besides the high-powered film industry, there's also the high-powered world of finance, and for some people it's a little intimidating to have that much commerce surrounding you at all times. But there's some great night clubs, and obviously, some great film festivals.
And of course, the excitement of being near the entertainment capital of the world!