I was an early mobile adopter. Started with CellularOne, Sprint, then AT&T, along with T-mobile. Have collected about 12 SIM cards around the world, pay as you go. These days, look for a good website as much as pricing. Recharge over the web is a great newer feature. downside is these days most SIM cards die after a month or more without use., even with a + balance on the account. They recycle your number.
Cingular went from worst to probably a close second for Mobile reception. After it bought AT&T, somethings improved. As you can tell, TMobile has worked out better for me. for price, performance and global flexibility.
Struggled with them all for coverage, but have had good luck with GSM more recently as more shared networks exist as in Europe. All companies have their ups and downs with towers and reception, so it depends on where you live and what exists.
My suggestion is to borrow a friends phone over a weekend (when calls are free) and see how it works from your home and office. (in California there is a 30 days/30 minute return policy with most vendors)
I agree with the 10 miles range offshore. Generally better range at night though. The US phones also work several miles into Mexico.
Tmobile has more acceptable international roaming, due to its size.
When I was in Thailand, found GSM works while on the boat at all the sites we hit.
GSM does not really exist in Japan or Korea. It is by far the most dominant standard world-wide with ocer 75% of the market. A reason why the USA adopted it last, (arrogance) thanks to our Companies that invented the market and screwed it all up. Today, Europe really runs the Mobile Handset and station Market.
What I frequently do is buy a local SIM card and use a spare phone to be able to accept calls from home on T-Mobile. I have used it all over Europe and Southeast Asia.
People I need to call, I either dial local or I SMS overseas.
SMS proves to be much better and cheaper than talking international, but from there I use several calling cards. Also, you can send an SMS with one reception bar, try talking on a single bar !
Hope this helps