Good luck when you decide to cancel it though, Three is a pain in the ass to leave with their hour-long questionnaire and legalese. I really liked their service but it was so annoying to leave I don't think I'll ever use them again. I'm a giffgaff-ian, by the way.
Ive been with giffgaff for a few years now and recruited family and friends to it as well. £10 pm with practically free calls it has been great. But with giffgaff's odd venture into payday loans as well it does not seem like a community driven company any more, hence why I might move to Three's PAYG (as I hate commiting to subscriptions)
https://www.giffgaff.com/money/loans
Interesting, I didn't know giffgaff was actually directly related to O2 besided using their network. Out of curiosity, I tried to find some information on Three and got a little surprise: "In January 2015, Li Ka-shing entered into talks with Telefónica to buy its British mobile division O2 for around $15.4 billion. In March 2015, Li Ka-shing confirmed it will be purchasing Telefónica's UK mobile division for £10.25bn subject to regulatory approval by competition regulator, Ofcom." So it seems Three is going to own both O2 and giffgaff by buying Telefónica?
The PAYG sim deal is cheaper (15GBP per month for the 'All in One 15' add-on) and works abroad immediately. When looking at their blog it seemed you had to wait for a month to pass for the Feel at Home to actually work.
I found the speeds were horrendous in NY also. But still better than roaming with my actual network provider.
"Feel at home" also works on pay as you go, and you can get a one-off, no commitment allowance that will provide it for as little as £10 (500MB + 300 minutes + 3000 texts, I think).
I've never felt the need to go for a contract in the UK, as PAYG is so cheap on Three.