I currently have Sprint, bought them before it was Sprint however since AT&T Wireless went downhill after Cingular and they wanted me to switch contracts over and I decided [censored] that [censored].
Nextel has proven to be nice and reliable for me. Everywhere I go I tend to get a great reception, except certain locations but noone tends to get any. I have the Motorola i205 which is now discontinued, but it has a real loud speakerphone, so I don't have to hold the phone to my face while driving, I can set it on my lap or cup holder in the Taurus. Customer service has been great during Nextel and now with Sprint. I've delt with them a few times reguarding small things. Although I rarily use my Walkie Talkie, I basically got Nextel because the phone was $.99, free activation, free shipping/handling, and so on.
Generally I really like the phone's durability. Sure it is old, and has no colour screen, but when I toss it across the room, it doesn't break
To me a phone is a phone, as long as I can call people, and tell them something, I am happy.
My views on other wireless providers: Cingular - Just crappy ass reception around here, same with US Cellular. Verizon - Last i heard of they don't use a SIM Card, so when upgrading comes around, gotta transfer ALL the numbers over. Sprint - The old commercials were annoying with that douchebag going around in his stupid trench coat. I kinda try to go for the provider with the least annoying advertisments...
So hopefully this can be useful