I bought a SIM for the US. For my purposes (occasional emergency use of talk and data multiple times a year), I found H20 wireless the best deal. I can often get by with less than $1 spent in a trip (if I have wifi 95% of the time). To keep my number costs me $40/yr and as long as I refill every 90 days, my balance keeps rolling over. Because I primarily use wifi, waiting until I get back to my room at night to post stuff, I don't use much unless I am on longer trips (particularly road trips) where I just want more freedom.
So when I go for longer trips, I simply switch to the $20/mo plan which gives me 1GB of data. That is sufficient for surfing and mild social media when I am out of wifi.