I live in a beautiful valley in a rural area in southern Missouri. I used to have an Alltel phone with tethering. I was getting EV-DO at home. When I roamed to other areas (San Antonio, TX and Minneapolis, MN), I could still get data, but it was 1X-CDMA (153 kb/s). It has never affected my phone bill. People visiting us, with AT&T or T-Mobile service, were unable to send or receive text messages, or make or receive calls, while they were here. It was Alltel or nothing. I had very few complaints with them.
I have, at home, a Cradlepoint MBR-1000 router connected to a Alltel USB-3G module (Pantech UM-150, to be more precise). As it is not roaming, I get full EV-DO Rev A. Data access for the module is about $60/month. If they have a bandwidth cap, they aren't enforcing it. We routinely suck down over 10 GB/month. I have an external antenna, on an external mount, which helps tremendously (a Wilson antenna, acquired from a local truck stop). Without the antenna, I'm lucky if I get 1 bar. With it, I get 4. The connection frequently flirts with 1 Mb/s inbound, with < 100 ms latency. DSL and Cable modem are not an option, at any price, so it is 3G, satellite (horrible latency) or dial-up for us.
I was very disappointed when Alltel was acquired by Verizon, with which I have had "negative experiences" in the past. I now have a Sprint smartphone (HTC Touch) with Phone as Modem (tethering). It has excellent 3G connectivity when I'm at work. When I go home, I have to roam to Verizon, so I'm back to 1X-CDMA speeds. The tethering is mostly used when I'm away from home, so this isn't such an issue. Roaming typically does NOT affect my phone bill. I haven't completed my 2-year contract on the data module, which is why Verizon is still getting my money on that count.
My dad is a truck driver. He uses AT&T. He has an external antenna on his rig, which gives him greater range. There are parts of Montana (mountains) and the Dakotas where he has trouble getting a signal. This is particularly true when passing through Native American Reservations in the western US. He doesn't have a data plan.
My advice: get a data module from Verizon (since their acquisition of Alltel, they do have the largest network), an MBR-1000 and a good external antenna. If you really want to get fancy, buy/build a directional antenna and get used to aiming it (it will be manual; I know of no system for automatically aiming a directional cell antenna; there is a market looking for a solution). There are places where you will not be able to get a signal, but a good antenna will shrink those areas. A signal booster will shrink it further. Verizon has roaming agreements with most other CDMA carriers, so you won't get screwed on your bill from roaming charges.
Oh, and stay the hell out of Canada. Nothing against Canada or Canadians (Vancouver Island was beautiful, and we met some very nice people on my last trip) but the data roaming rates in Canada are little more than legalized rape. I managed to rack up over $20 in roaming charges just reading one article in LinuxMag on my aformentioned HTC and briefly using Google Maps (with the GPS in the phone) to find our hotel.