Amazing variety of whitewater from easy to sketchy; day-runs to committing multi-day gorges.
When to go
The runs in the north of the state tend to work earlier: March to May. Further south (the High Sierras) work from mid-April onwards, with some of the high altitude hair runs only coming into condition in August. There is always something to paddle, and the area has excellent online river telemetry data so it it easy to work out where to go to catch the best water.
How to get there
Fly into San Franciso or LA. Virgin is a good carrier with kayaks.
You will need your own vehicle -- generally easy to hire although can be problematic if you don't have a driver over 25.
Unpredictable. Take kit for both warm and cold weather.