I've been using Project Fi lately and have been surprisingly happy with it. It's worked like a charm in Singapore, Taiwan, Japan, Argentina, France, Canada and the Netherlands for me over the last few months. I activated my SIM in an old Nexus 6, but use it installed in my unlocked iPhone 6 Plus without any issues.
Typically, the device associates to the local network with full LTE coverage, but data rates were throttled to 256Kbps. Now that the Three announcement comes with a lifting of that throttling cap, it will be even better!