I beg to differ. For starters you are piggybacking on O2's networks so goodbye to 3G. I was there last year, won't go back in a hurry.
I get perfectly good 3G signal in my area with both a normal O2 sim and a giffgaff sim.
Second, if you use up your goodybag allowance you cannot top up and have to wait until the following month to purchase another.
Where did you get that from? Total lie. If you do not renew your goodybag before it runs out, you can still buy credit, and you have to wait 12 hours before you can buy another one. Not a month, 12 hours.
Third, thanks to cashbacks, T-mobile has offered better deals: I currently am on a year's contract, which includes minutes, texts and data AND tethering for which T-mobile is paying me. No tethering allowed on giffgaff.
But you're stuck in a contract, and you aren't going to be making a profit from those cashbacks.
A £10 top up on a second free T-mobile sim gives me a year's free internet for my MiFi. Unlike some networks, T-mobile also does not sting you with overage but lets you surf and email for free if your data allowance is used up.
I'll give you the tethering thing, but then, I can get a second giffgaff sim and pay just £5 a month for 500MB on a data only plan, and make the money back from the payback. As it happens, I personally just use WiFi.
And giffgaff has no data allowance at all on the regular smartphone plans. It's all unlimited with no fair use.
Also remember that, not only do you get paid by getting friends and family on giffgaff, but you get unlimited free calls to other giffgaff users, too, even if you have no goodybag or credit.
And one more thing: when I first got my iPhone 4, I used T-Mobile, and it took me days to get my internet activated, because it wouldn't work by itself and the customer service is utterly useless. Not only that, but they made me pay for a micro sim! Then when it did finally get working, the actual coverage I got was terrible in my area, and when it kept roaming to Orange my data would stop working (this is what's supposed to happen, apparently - no data when your phone roams to Orange, and the same applies if you're on Orange and you roam to T-Mobile).
I moved to giffgaff after I was sick of that crappy service and got the whole thing set up in 20 minutes. Any problems after that were addressed very well by both forum users and agents. Not a single real complaint about giffgaff at all during my year of using them.