I'm actually planning to switch away from Giffgaff soon!
(With the Topcashback rebate, Three's basic plan works out £5 a month for me, with enough texts, minutes and data to cover everything I'd want - and I've had a few irritating outages with GG other networks don't have.)
There are a few minor downsides to GG: the voicemail greeting is locked on one identifying itself as Giffgaff, rather than a personal greeting like other networks, and no iPhone tethering allowed without a jailbreak and dodgy tricks.