I have to say, when I visited the UK from the US last fall, for the first time with a smartphone, I was amazed at how atrocious the mobile coverage was. I did my research beforehand and went with Vodafone as I expected the best coverage. From the time I activated my phone in Bath, through 3 days driving through Wales, and then up the motorway towards Edinburgh, I got GPRS and EDGE if I was lucky. I think I only occasionally saw 3G and never saw 4G or LTE. I was very convinced that my unlocked iPhone 5s was somehow not able to use Vodafone's faster bands in the UK. It was simply unusable for navigation or even the most basic tasks, and I was very happy with my choice to bring a standalone satnav device as well as the phone.

Then I arrived in Edinburgh, and, bam, 4G -- which persisted back to Newbury and London.

In the US, particularly last decade, we felt that we had been left so far behind in mobile technology compared to Europe, but my coverage woes this last trip made me feel somewhat better-- as did the realization that roaming charges were still a thing in the EU (having assumed they must have gone away ages ago).

The best coverage I experienced was in Iceland, where even the most remote-seeming canyon had top-notch data service, and our apartment for the week had 50Mbps FTTH service.

On a slightly more recent trip to New Zealand, I went with Vodafone again, and had much better luck with coverage-- I only lost coverage on the west coast of the south island which did not surprise me given the remoteness.