I know, i shoulda searched, but "O2" is too short to search for :(
I've just switched from T-Mobile to O2 (thanks to the "Free SIM Card" offer, although i've got no "free" £10!), and i'm really happy with the 300 free texts for topping up a tenner etc, i just want moar! :P
I've seen they do deals on getting 10% of your topup back (done), the O2 "Treats" thing where you get a ring tone (oh yay!) and i've done the "tell us your details for £2" one.
Anything else you could recommend?