Data roaming used to be expensive and inconvenient, but thanks to the EU regulations, roaming in Europe is much more reasonable (at least while the UK remains part of the EU).
If you want to use your own SIM in the US as well as Europe, 3's Feel at Home plan looks like an excellent deal -- you don't have to pay extra when you travel, you just use your usual data allowance.
But it doesn't always work on the phones I travel with; annoyingly often, the phone status icon tells me I have 4G from T-Mobile US, but I won't actually be online.
I've found it works better in Windows Phone 8.1 handsets than Windows 10 Mobile or Android phones. In an early Windows 10 Mobile build, I got a message saying that my Lumia 950 XL couldn't connect because tethering isn't supported. Presumably, the Edge browser fooled 3 into thinking I was using a Windows 10 PC. That message went away in later builds, but I still couldn't get connected. I switched the SIM back to my Lumia 1520 and got straight online.
After some lengthy troubleshooting with 3, we figured out the problem. An LTE contract with 3 doesn't give you LTE roaming. Windows Phone 8.1 seems to drop back to 3G automatically, which often means AT&T rather than T-Mobile. (On an unlocked phone that was originally from AT&T, I got a connection much more quickly by switching onto AT&T manually instead of leaving it to automatically pick up T-Mobile and then waiting for the connection to fall back, but after that, the connection happened quickly each time).
Most Android phones let you turn off LTE, which also solves the problem. When I first ran into the problem, Windows 10 Mobile didn't let you pick the network or turn off LTE, so my 3 SIM just wouldn't work at all.
On a recent US trip, I swapped the SIM back to a Windows 10 Mobile phone running an Insider build to see if things had improved. You can now choose between 3G and 4G and pick networks manually. I had to do both to get connected; with 3G and T-Mobile selected, I got online with an Edge connection (2.5G rather than 3G). After 10 or 15 minutes that had improved to H+ (that's enhanced HSPA, the fastest version of 3G before you move on to 4G). But the next time I checked, the connection was back to E, and it continued bouncing between E and H+ all the time I used the phone.
That's better than it used to be; I can at least get online. But the constant renegotiation makes for an annoyingly bursty connection, and problems I just don't have with Windows Phone 8.1.
If you don't want to deal with all of that, especially if you're a business sending employees abroad, or if you want a local number to make it easier for US colleagues to call and text you, one option is to get a SIM-only contract from one of the US networks.
You can pick up a StraightTalk SIM at Walmart that includes 8GB of LTE data for $45 a month, and you don't need a US credit card for that.
If you don't make frequent enough visits to the US to make that worthwhile, a temporary SIM like ZIP SIM is a little more expensive (there are shorter, cheaper plans but it's $50 for 2GB of data over 30 days, or $55 if you want calls and text as well), but very convenient.
There are plenty of temporary SIMs; ZIP SIM stands out for how easy and well-thought-out the activation process is.
The SIM is pre-cut in all three sizes from nano to standard, so you just press out the size you need. When you put it in your phone, you just send a text message with your current US zip code to the shortcode in the instructions. The welcome message provisions your device and, helpfully, tells you what you phone number is. I didn't have to do any manual setup at all to get data working; if you do, you can text the word APN to get details of the Access Point Name to put in your phone settings.
Connectivity in the US varies widely (driving up the Oregon coast with a personal hotspot on the Sprint network left me offline for hours at a time). I used the ZIP SIM in urban areas in two different states over the course of a week and data speeds and coverage were both good. If you need longer than the plan you started with, you can top up the SIM online.
The obvious disadvantage of travel SIMs is that you don't have your own number (although messaging services and Skype make that a little less inconvenient these days). Another problem you can run into is that the numbers assigned to travel SIMs may have been assigned to other people in the past. I didn't have a problem with ZIP SIM, but we had SIMs from other US MVNOs that received messages about the outstanding debts of whoever had the number before.
But you also don't have to worry about roaming costs or fiddling around with your settings to get connected. I've just discovered that I forgot to put my phone back on 4G the last time I came back from the US.
European Commission ends mobile roaming charges
Consumers will now be charged for their regular phone plan while roaming throughout Europe, with wholesale pricing capped at 3.2 cents per minute for voice calls and 1 cent per SMS by 2017, and data €2.50 per GB by 2022.
Here's what I learned going from Windows Phone 8.1 to Windows 10 Mobile, to Android - and back again
Windows 10 Mobile is getting much better, but so many of the features I rely on are still only on Windows Phone 8.1.