Short Answer: It’s very good.
Long Answer: The cell phone coverage in Korea is amazing. I read somewhere that Korea has the number one or two spot best coverage in the world. The technology is a generation or so ahead of the US- probably because of FCC stuff. You can get cell coverage out of the city and under four or five stories underground. I have nearly full bars all over Korea. Korea has very little land and half the population lives in Seoul and the suburbs.
The big three cell phone companies are: KTF, LG Telecom, and SK Telecom
Will I be able to get a cell phone that I can call the US?
Yes. It’ll cost a bit more. It’s better to use Skype for free. The cell phone plans are different here than in the US too. They are generally on a minute US basis.
Will I be able to use my Korean phone when I move back to the states?
I hear some of the newer phones do (they accept sim cards), but I haven’t tested it personally. The US also runs different uses a different protocol than Korea. I recommend just getting a cheap phone here for as long as you are planning to stay and trade it in every two years- if you stay here longer. That’s what I do. So much junk happens in a year to your phone that is better to trade in every two years.