i normally just buy a sim for my phone for the time I'm in the US. I don't use it much so only put a few dollars of credit on it but I was caught out on my last visit because it seemed that the provider had sold numbers to suppliers and I was receiving about 15 advertising texts a day. The effect was that each time my credit was reduced and within a couple of days had been completely spent. It had never happened before so maybe it was a one-off but I'll be checking the situation before I do it next time I'm in the US in a couple of weeks.

Might be worth bearing in mind for others.