I've had Airtouch (Verizon), Sprint, AT&T and finally T-Mobile...in that order.
I can't comment too much on Verizon since it was so incredibly long ago when I used them. A friend still uses them though and he talks on the phone like a woman and beats the piss out of his phone. He's satisfied with their sound quality and customer service, and he's the type that's never satisfied with anything.
Sprint really did sound great compared to all of the other providers I've used. Also, I'm not sure if it was the phone or what, but the sound quality wouldn't start degrading until I hit 1 bar of signal. Any higher and it was perfect. Their customer service sucked bellybutton and it was a pain to call them for anything. At about the time I left service with them they were switching to a system where you'd have to pay $5 to call customer service for certain issues. How f'ed up is that? I would never have switched though, except I moved into an apt that I just couldn't get signal in. When I quit, they charged me for an extra month of service since I was two days into a billing cycle. When I spoke to a manager to try and get that prorated, he basically told me to piss off because why should he care about me since I'm quitting the service.
AT&T generally sucked, but was OK for a cell phone. Using it as my primary and only phone though wasn't optimal. Customer service was good when I called in for the most part. Oddly though, they refuse to tell you how many minutes you've used in a particular billing cycle. Not sure if they just won't or if they some reason don't have the technical capability of checking that. It was very annoying. Switched because I went to work for T-Mobile.
T-Mobile has been somewhere between AT&T and Sprint in terms of sound quality. Definitely closer to Sprint though. Can't really comment on customer service, since I haven't found any reason to call. I know they've got a big focus on giving good customer service though, so I can say they'd be the opposite of Sprint (or my experience with Sprint). In CA your coverage will be the same with T-Mobile or Cingular since T-Mobile uses Cingular's towers down there.
Also, the GSM technology is cool. Uses a smart chip to store all of the acct info, so the service goes with the chip. You could literally swap phones every day if you cared to without needing to call up your provider. The smart chip also has memory so you can store all of your contacts on it and they'll move with the chip
For coverage, check with friends near you and see what their experience is. When you get a phone, make sure you find out what the buyers remorse period is for returning the phone and cancelling the service. If you get the phone from anywhere but a direct dlr (store owned by service provider), make sure they'll take the phone back if you decide to cancel service during the buyer's remorse...and get it in writing.