When I was traveling to the USA with my European phone (iPhone 4S with 3G) last year, I took a prepaid card at T-Mobile and it worked fine for me. The vendor told me the 3G *could* work but since they were deploying the network (at the same time they deploy LTE), I shouldn't count on it (I was in SF then).
3G worked when I exited the shop and put the new SIM card for about 20 minutes, then it fell back to Edge for 99% of my 6 weeks trip.
I traveled on the east and west coast and only had 3G another time, at the top of my hotel in Manhattan at night, just after Sandy. Since most of the towers were down (because of the hurricane) that was a quite good surprise and I could Skype my family to tell them I was OK.
Then, when I left, my iPhone told me I was connected to AT&T 5G. Never understood why, but it was fun to see (should have taken a screenshot!).
For a 50$ plan with unlimited voice, text and data (of which 500MB of data high speed then low speeds) that was quite nice. Should they get a better high speed coverage, it would be definitely the best you can get when traveling in the USA.