I lived in San Diego for a year. It wasn't for me. (My stint in San Diego ended after I visited San Francisco and moved there shortly thereafter.)
The weather is damn near perfect year-round and the sunsets are beautiful. There are also plenty of beautiful people and there is much beautiful scenery. I just wan't into the beach culture thing, so there wasn't much there for me. Also, the Navy and conservatism pervades everything, so you'll probably be an outcast if you don't wear the flag and vote republican. (I was once cold-cocked by a Navy squid one night while walking down the boardwalk minding my own business. All I could see were the silhouettes of Navy personnel, and I heard one say, "Hey! Watch this!" The next thing I knew I was laying by the side of the boardwalk looking up at the stars. I figure he probably picked me because I had long hair and was chatting with a homeless guy at the time, so he figured I was just a bum. I still have a chipped tooth as a memento of that event. Wow, this is a really long parenthetical, isn't it?!)
There are oases of interesting culture, however. Ocean Beach has a bit of a more bohemian feel to it, and San Diego in general has a thriving medical/biological research community. Then again, it's really nice to just forego culture once in a while and hang out at the Lahaina Beach Club and drink pitchers of beer on the deck at sunset. (Sometimes a pitcher is worth a thousand words.)
I think a large part of my dissatisfaction with San Diego had to do with where I was in my life at that point. If I didn't have the choice of living in San Francisco or Seattle, San Diego might be a decent third choice.
By the way, I wouldn't recommend going to Tijuana. I found it highly depressing. It's a misanthrope's nightmare come to life...
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"And Wirthling isn't worth the paper he isn't printed on."