I moved over to Giff Gaff this week, so far so good. I get a MUCH better signal around Manchester than I ever did with Orange. £10 a month for 250 minutes, unlimited texts and unlimited data.
HOWEVER.
Giff Gaff have said they can no longer afford to support unlimited data at the current cost. They're giving customers three options.
1. £10 a month for 750MB
2. £13 a month for unlimited data
3. Both become plans and you can pick either
It's being discussed on the GG forum at the moment before a final decision is made.