Being an O2 MVNO, they're hopefully not going anywhere (bad) soon....
gonna port my number from T-Mobile > giffgaff at the end of this month, as having tested it for a few days in my 4S, it's consistently faster than TM in pretty much every data test I've ran. I've still got an old O2 PAYG SIM I use in a combat phone when I go on the lash, which always gets a decent signal, so it's a no-brainer really.
Really can't complain at 10 quid a month for 250 minutes, unlimited texts and unlimited data, so just go for it.
Easy to setup an iPhone for data access on giffgaff too, as they seem to currently get O2 APN settings.... quick trip to the forums on GG has plenty of help.