i have AT&T. I've been w/them since they were LA Cellular (3 years ago).
I have been rather pleased w/their service. coveragewise, i think AT&T is the best because of the national coverage. i tend to have a strong signal wherever i go.
as for the plans, they are great right now. for $29.99, you can get 250 daytime minutes (M-F 7am-8pm, i think), and 500 night/weekday minutes (M-F 8pm-7am, all day Sat & Sun, and some holidays). You get free voicemail, call waiting, call forwarding....etc etc....and you can call anywhere in CA w/o long distance charge. you can roam in parts of nevada & arizona (tahoe & vegas!). the network that AT&T is on is better as well.
w/the at&t phone, u get dual band -- digital and analog. w/this u get coverage in more places. w/pac bell pcs, you only get digital.
i sound like a salesman.