StraightTalk is a popular choice for folks in the US wanting to save some money every month. It is $45 for unlimited voice and messaging. Data is "unlimited" but you're actually capped around 2GB. I think you need to some weird T-Mobile SIM card swap to get data working, which for a visitor to the US is impractical.
Depending on your length of stay you can consider AT&T and T-Mobile prepaid. T-Mobile does
not have 3G that works on the iPhone in most of their coverage areas, but they do offer true unlimited data (since it only works on Edge, it won't be very fast). Configuration is easy and often they will do it for you in the store.
AT&T prepaid has an unlimited plan, but the data that plan comes with does not work on the iPhone. You would need to buy buckets (prepaid amount) of data which is $25 for 1GB. If you tell the store you want to use it with an iPhone, you won't sell it to you or you will, but won't help you further. This is because AT&T's policy is sell the iPhone with a post-paid (contract) plan, and for a visitor that's not practical. If you want to use AT&T (which has 3G everywhere), you need to follow instructions here:
http://www.dargadgetz.com/how-to-use-iphone-5-on-att-gophoneprepaid-with-4g-data-speeds-payasyougo/
Just a heads up when buying a prepaid SIM card in the US. You can use cash or whatever to pay. When they ask for your name, if you can choose to give any name you want. They're not suppose to ask for ID or anything. No need to identify yourself as a visitor