I have to agree w/ carioca. I had Sprint with SERO for a while and had no problems, except none of my frequently called numbers had Sprint, so I went hundreds of minutes over for a few months, so I left. I have an iPhone now, and it's pretty hard to say "I think VZW would have done better" or T-Mobile, or Sprint, etc. Every carrier would provide a different way to get something done, and I actually think that Sprint and it's fairly liberal ToS and pretty great customer service as of late would be a great place to try to create the next iPhone killer.
I'm just a little tired of random and thoughtless carrier bashing. Each national US carrier provides its own benefits.
Verizon Wireless: LG VX 4500, Motorola Razr
T-Mobile: BlackBerry Pearl 8100 (returned)
Sprint: Motorola Q, Blackberry Curve 8330, HTC Mogul
Verizon Wireless: LG Dare, BlackBerry Curve 8330
AT&T Mobility: Apple iPhone 3G, Blackberry 9000, Nokia e71-2