essentially (for the southern ca. area)
in terms of coverage and convenience (i'm talking from experience)
i can rate the carriers as follows from best to worst.
Verizon / Cingular (tie but verizon rapes cingular once u leave socal)
ATT
T-mobile
Nextel
Sprint
I have worked for pacbell, tmobile, and cingular... I am unbiased and I know that cingular used to have a shitty rep... but they have invested more than any other carrier into increasing cell sites in ca(over 1 billion) and now have the best coverage IMO, they also have a pretty well setup gprs network... Hard to beat tmobile though if u do MMS messaging... they have unlimited data xfer for 20/month... that is SICK.