Okay I sell cell phones and I go to UC Irvine and have pretty much experienced or have friends/customers/workers that have service around this area.
Cingular. Was absolutley horrible a year ago but several new towers. Works good on campus and anywhere near campus of course, but not good in Newport Beach, Corona Del Mar area. (Friends/friend's roomates have experienced) Expect to see LOTS of Nokia phones on campus, so if you dont liek to have a phone everyone else does...
Team Mobile = Cingular (they use the same towers).
AT&T. Average, pretty much anywhere on campus, Irvine, or neighboring cities. Kinda spotty in parts of Irvine though. (ex-roomate had AT&T)
Sprint. Spotty everywhere. Doesn't work well in the dorm rooms. Works avearge in Irvine and enighboring cities at best. (dorm room friend from 2 years ago has Sprint).
Verizon. As someone said before, doesn't work right around the ICS/Engineering buildings. Spotty in some places on campus. Works well in Irvine in general and neighboring cities. (Cingular & AT&T friends have no reception at my place, but my Verizon does... fyi I live right in between the 405 and 5 on Culver.)
FYI, no service will work in particularly great as you start moving west toward the coast. (Huntington Beach, Newport, Corona Del Mar, parts of Irvine, etc). The closer you get to Santa Ana, the better. =P
In general, Verizon is good (they leech off of everyone, 2 digital frequencies + analog = tri-mode), AT&T and Cingular (TDMA, which is going to be phased out or GSM for AT&T/GSM for Cingular) have been doing not so well lately, Team Mobile (GSM) is doing pretty well and might merge/buy out one of them (AT&T or Cingular), Sprint is slowly improving (but its all digital, and uses only one frequency which really bites.. analog roaming is expensive).
And CDMA provides better reception than GSM at the moment (you can take them farther away from the towers) but the phones are less full featured usually, but you said you dont care about that. GSM phones are the only ones that would ever work internationally also (if they are Tri-Band)
And don't go to Anteater Forum. It's horrible, its pretty much juast liek a group of 5 friends or so replying to each other the entire time.
Last but not least, I don't know anything about nextel.