Well, I guess I'm the voice of dissent here, but I like Sprint. I went through a half dozen carriers years ago, and I've got several friends and former co-workers who tried various carriers, and from my experience the best nationwide coverage for someone who travels frequently is Sprint PCS, primarily because there isn't any such thing as "roaming" or "out of network" if you're in a metro area.
I've had Sprint PCS since 2000, have performed seminars in at least 200 cities throughout 38 states, and the only time my phone was rendered useless was during a drive from Fargo, ND to Karlstad, MN, Population 38 (I was actually giving the tech seminar at the phone company in northwest MN -- if that gives you an idea of how remote I was to be left without service). Of course, I could have subscribed to the optional off-network plan for another $10, but this would have been the only time I would have ever used it. I would provide a disclaimer for my personal experiences though by saying I didn't work the Pacific Northwest, my territory was mostly east of the Mississippi, though the few trips I did make out west (several cities in CA, AZ, TX, OK, CO, ND, SD) all went without any service interruptions.
For anyone who has ever lived out of a suitcase, the number of cell phone calls you find yourself making is obscene. In one day I'd have to call corporate home office, my travel agent, my arriving hotel, a transport service in my arrival city, plus calls back home to friends and family. I'd use at least 60 minutes every day, from anywhere in the US, and on Sprint I have never, ever received a bill in excess of $55. I've never had a bill over $55 even after including data services (laptop/e-mail in airports) and blowing around 2500 minutes round-the-clock in a single month.
For those that complain about Sprint's Customer Service, I say goto
www.sprintpcs.com and do it yourself. Whether you want to change your plan, pay your bill, add/remove a service a la carte, you can do everything through their web site. You can even upgrade your phone firmware with codes and guides available though Sprint's site. One reason I switched to Sprint from my last carrier (back then, Airtouch) was that I could walk into a Radio Shack and pay cash for the phone, take it home, and activate it myself without ever having to sign anything. If I should happen to lose my phone, I could either call Sprint and get a replacement within 48 hours, or in an emergency I could go to any Radio Shack or Sprint dealer in the country and buy a replacement phone and transfer my service to that one myself using the Sprint web site or following the manual included with the phone.
As far as specific model phones, I'd take anything from Nokia or Motorola. Stay away from Samsung products, period -- cheap Korean POS. LG Electronics (Goldstar/Zenith) isn't much better -- more cheap Korean POS.