I use Three because apart from Canada their international roaming deals are way better for my purposes (as in: free all-I-can-eat data in almost all the countries I visit). The problem is the holes in the network of bilateral roaming agreements between carriers, which are deliberately opaque to customers.
If you do a lot of travelling Vodafone is usually better. Global Roaming Plus (https://www.vodafone.co.uk/cs/groups/public/documents/webcon...) covers more destinations than 3 'inclusive' and nearly every country in the world outside of that is £6/day. Whereas with 3 you're stuck back to £10/MB nonsense if you go outside of their roaming countries.
