I am from Long Beach, California.
I started out on Cingular with the 8700c. I got it unlocked and switched to T-Mobile to save money (I was paying over $110 a month for what I could get for $60 a month on T-Mobile) and I have also updated operating system software and themes (got them from Howardforums.com). I have since recouped my savings by moving to T-Mobile. Cingular custer service was OK (never had a proelbem with them). T-Mobile custer service is great (less times on hold, don't have to repeat myself as many times to get my information across).
I know I will be with T-Mobile for at least 5 1/2 years (TTR or time to retirement).
To recap, T-Mobile has excellent voice/data plans and better customer service. The only regret I have is waiting so long to switch.
Oh yeah, hurry up and get the BB 8800 our for T-Mobile, I need an upgrade fix.