I've had Verizon, T-Mobil, Nextel and Sprint.
They are all work phones so I can't speak to the billing issues. From a functional standpoint, Sprint is the worst. It is so bad that we consider our phones not functional work tools. Sprint phones:
1. Have low signal one second, then switch to 5 bars the next.
2. Calls started at high signal levels drop off and the phone displays the infamous "signal faded" message.
3. Speaking of item 2, the phone has a dedicated "signal faded" message to tell you why it disconnects so often.
4. It roams often. It is in roam even at our local Sprint store.
5. Calls many times have annoying echos and distortion which makes it very hard to converse.
I wouldn't want a Sprint phone if it was FREE... well to me my phone is, and I don't want it. We are forced to use them because of a corporate directive.
I have a T-Mobil phone at home and it is better, but I don't use it much.
At work we used to have Nextel phones. Big and clunky, they always worked well and have the best speakerphone of any. The PTT feature works GREAT. Nextel is a good business phone.
I think Verizon is the best as far as coverage. I've heard that they have high rates and have poor customer service. Perhaps they have so much business they can afford it.
The above are my opinions only. YMMV.