IA signed me up to GiffGaff when I got an 2nd hand iPhone last year, as it was by far the cheapest way to get unlimited data and texts, without a contract. Got my existing number migrated over to the GG sim easily. Hardest part was telling O2 to give me my MAC.
I don’t really use many minutes, normally get 300 with the £10 Goodybag. I like the fact they will tell you what is currently the best deal for your usage. E.g. if you don’t use the bundled minutes, they text you to say that you’d be better off just using PAYG next month.
I then moved to the USA but use the GiffGaff’d phone one when I’m back in the UK. The credit stays on the account, and I use that to buy a Goodybag before I return.
The only problem I have had with them was dealt with fairly well. In August they ran a ‘buy a goodybag and be entered in competition X’, which ended on the day I happened to choose to buy mine for a trip back to the UK. Their hamsters got tired due to the load of people buying that evening. My credit disappeared, but the Goodybag did not appear. Took 24 hours to get it sorted via their online help system, which is the only way to contact them. It cost me literally pennies to use some data when we arrived to email people, before it was sorted.