Giffgaff piggy backs on the o2 network and is subject to the old o2 fair usage policy that my about-go-expire o2 contract has (which is 20GB apparently).
I doubt it covers tethering as o2 don't like this, nor do they like streaming but they can't prove either.
So it seems too good to be true, but it is true. I know a few uk based giffgaff users and they're all happy. If I could afford to buy a handset out right or live with this for longer, id definitely make the move.