Hiya !
I'm not sure which city or areas of the US you intend to be at, but I'll assume one of the major cities or large towns. Coverage and service should be pretty good but as I'm sure you know, the further out you go and along highways in between places, there's usually no service at all. Last time I was in the US was feb 2005 in California around SF and the Bay area as well as about an hour inland, Stockton, Sacramento areas and found no service quite frequently when driving around.
I used the T Mobile pay as you go offerings which was a cinch, just paid up $100 on my credit card, they activated the SIM and I just popped it into my handset. As Vicky says, as long as your handset is tri- band and is unlocked it'll work so you can call up on the T mobile networks advertised domestic rates.
Depending on which city you are in, you'll get a SIM card programmed with the city code/dialling code of the city you're in, so that may be a factor if you plan on giving your US number to members of your family who maybe in other states etc.
Hope this helps
Cheers, DtM, West London & Slough UK!