Interesting that Verizon is more generous that was than Sprint. The Sprint one comes locked, and I think even if you get it unlocked for foreign use, it doesn't work on AT&T or T-Mobile. (although I haven't checked that.) I did ask Sprint to unlock it, though, and it worked flawlessly in the UK with 3G speeds. No lag moving in and out of Wifi, really, no problems at all. I was a happy puppy.
And when I brought it back the the US, it still worked fine on Sprint (at 3G) without swapping the SIM back, since the SIM is only used for 4G in the US.
I did have to update the settings so it knew to look at the new SIM, but that's automatic, you just click the "update" button in the settings.