I make about £20 a month out of giffgaff.
I work at a school, they have lots of international teachers etc. They all come to IT for help setting up their phones, skype, etc. to call home.
I don't force it, but I hand out giffgaff SIMs. Every time they activate one, I get cash which gets to my Paypal eventually.
And lots of them do stay on giffgaff because of the "PAYG / Monthly" thing so they can pick and choose and be short of money and knock down their package without obligation. Nobody ever calls for support, so they don't notice giffgaff's is only online.
They really need a "big bundle" though, that I can tether for mobile broadband. 9Gb isn't enough and even the unlimited package is subject to a tethering limit.
I also activate a lot of their cards for things like GPS trackers and GSM gate openers. Run it down on PAYG over a year from about £20, or stick it on the £5 a month one if you want guaranteed service.