Tried Three for two months from November last year. I spent the whole of December in Florida and their free use in the USA was wonderful with proper 4g data everywhere and a great signal. Sadly in just 6 weeks usage in England I had two complete days of network outage and further regular signal drops on both data and phone signal. I live in Manchester so no excuse for bad signal. Never going near Three ever again. Voted freezing cold here.
MoneySavingExpert.com doing a deal with Three at the moment - £16/month, 12 month contract, unlimited everything.
You might aswell just get a smarty SIM for a tenner with 30gb data on plus any cashback not that I would.