Given that I am planning to spend nearly four weeks in the USA, I thought it would be worth buying a local SIM card for data purposes. I wasn't particularly anxious to find out about Three's clear-as-mud roaming pricing via an enormous bill when I got back.
I ended up buying an 16GB Lycamobile "data only" SIM at the airport for $95 (incuding sales tax) - it lasts for 30 days. That seemed rather on the steep side (verging on vertical); I suspect they over-charge the incoming tourists for something which could be obtained more cheaply at a corner shop - but when you've been up for 18 hours and need internet to work out transport to your hotel, the "pay up and shut up" reflex kicks in.
They did at least have a SIM poking tool which they used to put the card in my phone (that's one thing I forgot to pack). Once again, the dual SIM capability in OnePlus phones comes in handy - I can leave the UK SIM in slot one just in case anyone sends me a text, but send all the data via slot two.
As you can see from the screenshot, it works pretty well, at least in the middle of SF. I'll update at the end of the trip on how well it holds up outside cities (and indeed when I move into other states).
Update: it does IPv6. Not a huge surprise, since many of the UK mobile networks are now doing this, and the USA obviously has many more people and an equal shortage of IPv4 addresses. This feature proved particularly handy when I was making some VoIP calls home - over IPv4, the calls had one-way audio; forcing the use of IPv6 solved this. VoIP is one of those applications that really needs a proper end to end connection with no NAT in the way.