I've got giffgaff, its great for me because it only costs £10 every 3 months.
If you top up by £10 you get free texts AND calls to all other giffgaff users. To get this you don't need to buy any goody bags.
Internet is 20p per day, max 20mb so highly recommend using Opera browser to reduce your bandwidth. Beware that it is 20p per mb above 20mb, so if high user it might pay to buy a goodybag.
Other calls just about to go up from 8p per min to 10p
Other texts about to go up from 4p to 6p.
I find that goodybags aren't worth buying for my usage, but as I have credit in my account all the time, I could buy a goodybag if needed.
When you top up by your initial £10, you get £5 thrown at you free as well.
Main trick as you can see from above is to get all your friends and family on giffgaff as well (I think you get £5 per referral as well).
O2 network as you say, not brilliant but generally usable although I don't use it for internet as yet so can't vouch for that.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates