* The monthly contracts (and higher value pay-as-you-go top-ups) come with "Feel at Home" roaming -- in about 12 countries worldwide you can use your inclusive minutes, texts and data with no international roaming fees, i.e. you get that all-you-can-eat data overseas. Countries listed include the United States; on one recent 2-week trip to the USA I was able to use my iPhone for data as if I was at home with no added charges.
* British phone tariffs do not charge you for receiving text messages.
* The all-you-can-eat is capped (small print) at a "fair usage" of 25 Gb/month. Tethering is disabled while roaming overseas on "feel at home", but not prevented at home.
* If you buy a smartphone from Three, they'll unlock it at any time, for free, on request.
* In non "feel-at-home" countries they'll give you 24 hours' data (capped at 500Mb) for £5. Not as good, but better than typical international roaming fees (at £0.5/Mb in the EU and up to £3/Mb outside the EU).
Those of us who are used to this kind of cell service boggle in amazement about how backward the US carriers are.
The final irony? Three is pretty backward and expensive compared to what Estonians get from their cellcos ... the UK is by no means the best world-wide, it's just a handy point of reference.