I think the answer to hecatae's closing question is really all about CPW slashing their margins with Vodafone for the data portion of their deal. That is going to be compensated for by the 2-year contract lock in.

This is not the most compelling proposition. Particularly in the light of not only giffgaff but also Three's PAYG offers. But if you want a new smartphone also then there is always Three's The One Plan to compare with, and that allows tethering if desired.

Like everything mobile networking, please make sure that the operator has adequate coverage in your area of home, work and play. Try their network out with a PAYG SIM first. That £10 top up might just save you hundreds of pounds of contract fees and a ton of heartache.