In reply to puppythedog:
I've just switched to them and seems good so far....
The goodybags can be set to automatically recurr, essentially putting you onto a contract, but you also have the option of just putting credit on the phone (not a goodybag) and using that (can be set to automatically top up your balance if it falls £3, and limited to doing it X many times per month). I'm currently mixing and matching to see what the best balance for me is.
Downside as someone has said is international roaming is very expensive and goodybags are only valid in the UK so any calls / data outside of the UK are paid for in addition to the goodybag.
Good thing is being PAYG there's no commitment so if its not working out for you, walk away with no contract issues.
You can't top up by card with your phone though - has to be either online or via a voucher.
Cheers,
Stuart