Personally I love California Adventure. I go in midday to get away from the crowds at
Disneyland, so I can eat lunch without waiting an hour in line!
Definitely ride the Maliboomer before they take it away. The launch is amazingly fast and the line is always short because it scares people (even though it's not that bad). Tower of Terror is similar, but a longer ride with a more intense feeling when you fall.
California Screamin' is an excellent coaster, decently fast and has on-board music. It's amazingly fun and worth up to like 45 minutes wait. Get FP's for it as much as possible, and ride it at night if it doesn't break down because it's double the fun!
The Sun Wheel scares me, personally. Might be fun if you aren't afraid of heights and enjoy the feeling of a constant car accident, ahah. When that cart starts moving I look for anything and everything to hold onto while I scream!
Most of the other rides are pretty fun too. People love Soarin' and Grizzly River Run, but I'm not as big on those. The Electrical Lights Parade is awesome and the Aladdin show is the best!
If you like Universal Studios, I think you'd enjoy California Adventure.
My advice is not to take a whole day with tickets just for California Adventure. Take a day or two to park hop over to it so you aren't stuck there if you don't like it.