As long as you won't be traveling to rural areas, T-Mobile is by far the best choice in California and elsewhere. Pricing is one of the big advantages---in most markets they're now offering 1000 nationwide anytime minutes plus unlimited nights and weekends for $39.99 + tax per month. Plus as others have mentioned, you can use any GSM phone as long as it's unlocked. Phone and plan prices are all good with only a one-year contract (Verizon, Sprint, and others now ask for two years to get the best deals). I've used AT&T, Sprint, and Verizon in the past and T-Mo easily beats all three in terms of value, customer service, and handset selection.

Just be sure to check coverage on the T-Mo web site if you'll be going somewhere off the beaten path. Most areas off the Interstate highway system and outside metropolitan areas are not covered, so I carry around a used Nokia TDMA/analog phone with prepaid service from CallPlus when I'm out in the middle of nowhere. CallPlus is a small price to pay for the security of having a phone that works *anywhere* there's cell coverage...only $10 for three months of service with a small amount of airtime included.