Correct me if I have missed something but it doesn't look very special. Other networks compare with the rates and Three at least definitely gives better coverage and speed than O2 in most cases.
GiffGaff was looking like something different to begin with but slowly it's turning into just another typical MVNO....
I will take issue with you in 2 respects davegr.
Firstly, the last rate comparison published which I have seen puts the order of network internet speeds as being O2 top, Vodafone second, '3' in the middle, T-Mobile 4th, and Orange bottom.
Secondly, 3GB of data on '3' pay & go costs £15, not £12-50, AND '3' explicitly ban tethering of any kind ( and tests show it CAN'T be done. )
When you buy a £15 Add On, which claims to offer unlimited internet, it will work on your mobile BUT NOT in a dongle or other device.
(This is different from the old Flat 12 and 3Pay SIMs which some of us still have.)
The new data only Goodybag on Giffgaff IS good, because of both it's price, and the explicit allowance of tethering.
(On '3', only the One Plan allows both unlimited data and tethering BTW. )