I used to work for a company testing games and applications on cell phones. The cool part was just that, at least for people who are really "into" cell phones, I got to try out pretty much every phone on every carrier. Honestly, Cingular was one of the better performers and that's why I'm with them today. In my opinion:
Verizon - Overpriced voice, data, and handsets but had the best data plans available and decent customer service, especially if you were on a business plan. If you could get a handset on the internet and unlock it then you'd be ok for a decent phone and like an evdo/bluetooth setup.
T-Mobile - Had the best selection of handsets and the best price offerings of anyone we tested but also had just about the worst service and worst support.
Cingular - Kind of a middle ground of handset choice/service/support, at least since the AT&T merger. Before that AT&T had the absolute best support (IF you had a business plan) and decent service. The odd part was that AT&T phones had an unreasonably high amount of confused calls (where two people who were trying to make calls at the same time get their calls switched up).
Sprint - Sprint had decent service and decent support but I have to say I hated every single handset I ever used with them and that's why I could never go with them. For someone who's into phones, there's not much you can do with them. If you try to hack them they won't work with 90% of the proprietary services Sprint offers. And as far as loading Java apps and stuff, all the other phones have some sort of Jar file uploader but Sprint makes you compress it or something in a weird way and upload it to some server and download it via your phones browser or something. I've heard of great prices/plans people have gotten and really bad ones as well. HOWEVER, if you're just looking for a phone to make and receive calls (not much more), then Sprint may be your best choice.
Nextel - Nextel really does provide great business phones because (a) there's not much else to do with them besides talk and (b) they're some of the sturdiest phones I've ever seen available to the public. Some of these tough Motorolas they offer are really tough. We've dropped them a couple stories and they're cool. I've dropped my Nokia 3 feet and cracked the lcd/severely messed it up. I don't think they're great for personal phones.