For some time to come I will be spending 2/3 of my time in the Bay Area and 1/3 of it in Fairbanks.
In the past I've had Verizon and had good phone service in that scenario, as well as excellent data service. (Verizon "roams" on ACS while in Alaska, and the service is probably the best in the area.) I didn't particularly like Verizon as a company, though.
Now, I have a new Galaxy SII and a monthly T-Mobile plan. I like the phone and I like the plan and all that, but, I have no data service of any kind in Fairbanks. That's not exactly a dealbreaker because I really only need voice and text while I'm in Alaska, but it is contrary to what T-Mobile's service maps show. They quite clearly show "partner" 3/4G coverage where I am, and across the whole Fairbanks area.
In Alaska, T-Mobile roams on GCI and ATT, depending where you are. However, I haven't had data service in any location, including the Anchorage airport.
I do have "Data roaming" enabled on the phone.