No idea about the data capping, but giffgaff topped the Which? list of suppliers. They piggyback on the O2 network. They're trying to increase their customer base, so on the PAYG (don't know about the goody bags) they give you a period (poss 3 months) free calls and texts to other giffgaff users every time you top up - very useful if, eg, your OH and DC are also on giffgaff.
we use giffgaff and can recommend it. All of us have the £5 a month goody bag which includes a certain number of calls, texts and data and if the kids run out of minutes, as we are all giffgaff they can still call us and text us for free in an emergency.
I use this offer and don't really notice a massive slow down after the 9GB. I listen to a lot of music on radio and youtube when I'm walking to work and there might be the odd pause but in general still pretty good.
Can't comment on how slow the data goes once you have used up 9gb as I have never used anywhere near that.
(Just wanted to add for those that don't know, you also get paid to use their forum! I only post on there a few times a month and sometimes earn up to £10 in payback a month - that gets paid twice a year as credit or PayPal!)
I've used Giffgaff for at least 3 years and never had a single issue. I love them and recommend them to everyone. Now I've got my mum, dad, sister, sister's friend, my ex, my ex's wife and my best mate on giffgaff too, which is fab as I get free unlimited texts and calls to them even if I have no active goodybag. Used the data when roaming abroad in Spain, Portugal and France with no issues at all too.