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I intend to travel to the US for a month, and I would really like to use a lot of data traffic during the trip, for browsing, maps, Skype calls, etc. How can I do that? Should I buy a SIM? Do I have to order one in advance, or are data SIMs readily available in the airport? Is there any website that can show me competing options and maybe compare them?

Also, I guess SIMs are linked to one specific carrier. Does it matter which cellular carrier I choose, or do all have similar coverage and do all support GSM phones? Does it change from state to state? e.g. do some of the more states, like Hawaii, have limited coverage for some of the carriers?

In short I am clueless here, and would really appreciate any information about this.

EDIT: meanwhile I have found this comparison between various data sim suppliers, seems very useful though I'm not sure how reliable or up-to-date this page is.

Well, turns out this is really easy - I just entered a store and asked for a monthly plan which includes unlimited data, and the clerk was happy to offer me a choice between a couple of available plans, without any requirement except a credit card (no need for zip code or credit check or American credit card / address). The store was T-Mobile, but I think other carriers offer similar deals as well.

Just be aware that the listed plan prices usually do not include sim cost or one-time activation fee. Still, travelling with an Internet-enabled smartphone is amazing, and definitely worth some money.

The sim card was activated almost immediately - I was able to data-surf while still in the store.

AT&T has PayAsYouGo SIM cards, you can buy them in the real AT&T stores. You just come in and ask the clerk to sell you one. It costs $20 (as far as I remember). Then there's a $20/200MB (not sure about exact numbers) data plan for this card which is pretty expensive if you really plan to use the internet extensively.

T-Mobile also has prepaid SIM cards that cost $3.00/day for voice/text/data. The data is 200MB at 4G speeds and then drops to unlimited at 3G speeds. The SIM costs $10.00. I do not believe there is an ID requirement (they might need your zip code into their system though), you can pay in cash, and they are available at T-Mobile stores. You can refill with a credit card via phone or by purchasing minutes cards in grocery stores and the like.

I haven't seen SIMs for sale in the airports I frequent, but I also don't look for them either.

Other major operators (Sprint, Verizon) are off limits if you have a GSM phone.

As a broad generalization for the US as a whole, coverage is equal for all carriers. A bigger concern would be getting coverage in less populated areas.

Was in the same situation a while ago, and found it impossible to find a prepaid sim for my phone in the states. Short answer is: it's easier use open WiFi hotspots instead of purchasing a prepaid sim card.

The long answer is that you can get really cheap phones for less than $30 on supermarkets, corner stores and petrol stations, but they all have a sim card build in and you wont be able to take it out and use it in your sim. AT&T and T-Mobile do offer sim cards you can use in any phone, but you won't get a sim with any prepaid plan and they are usually only sold in the "big" AT&T/T-Mobile stores, not in the small stands you might find in the mall. If you're lucky enough to find one make sure it also works in your phone - from what i have heard the T-Mobile sim cards don't work in unlocked phones.

The correct term is "SIM only Pay-as-you-go-plan" in the states, otherwise the store operator might not understand what you're looking for.

So if you really, really need prepaid and data you should use your own and get charged a arm and a leg for roaming fees. Otherwise turn off your cellphone and log in to the free WiFi near each Starbucks or fast food chain to check your emails.

I use a service from a company called Travelers Mobile.

They are Canadian and I am in Canada, but they ship anywhere in the world.

Basically they sell me, a US local prepaid no contract plan, delivered to my home in Canada, cheaper than if I buy it from a store in the US.

They currently sell US and UK plans. I have not used the UK plan yet, but I have used the US one several times. Works perfect.

I was in the US in may 2013. I checked with several operators, and the best deal by far was from T-mobile. I could buy a pre-paid sim card with unlimited voice, text and data for 3 USD a day.

If for a longer time, 90 USD a month could be high, but they sold for arbitrary number of days, so if you stay for a week or so, it's very convenient

T-Mobile now offers a 30$ prepaid international tourist plan with unlimited data, unlimited domestic texting and 1,000 domestic minutes of talk time, with a free SIM card. The main limitation -- apart from no international calls -- is that the plan expires in 21 days, so @Oak's answer still holds for longer stays.