I really like 3. I have a PAYG SIM and top up £15 a month to get their All in one 15 bundle. This gives me 300 mins, 3000 3-to-3 mins, 3000 texts and completely unlimited Internet access. The O2 network in Norwich seems pretty congested and slow in comparison to 3 which is fast even at the worst of times.
3's all-in-one bundles work on a queueing system. Each component of the bundle is counted as a separate item and the bundle buys you the three components. If, for example, you ran out of minutes two or three days before your remaining texts and unlimited Internet ran out (like I have several times), you can buy another bundle and the minutes will become available instantly, but the texts and Internet access will be queued up and become active when the current texts and data is used up or expires. I have not heard of GiffGaff offering that.
3 allows tethering on PAYG (unlike GiffGaff), which is a great perk and something I use quite often.
Whilst 3's customer services reps are based in India, when I have called before, the people I have spoken with actually seem to know what they're doing. A few months ago, their topup system wasn't working. I had £7.50 of credit left and explained to the 3 customer services rep that I wanted to get the £15 all-in-one bundle. The rep tried my card twice over the phone and it failed both times so he just said "ok sir, we'll just give you the bundle for £7.50, sorry for the inconvenience".