I'd been with Giffgaff for about 6 months, and in that time there was an uncountable amount of outages, some lasting up to 3 days. Their website and payment system was useless, I tried to purchase a £10 Goodybag; they took the money but I didn't get a Goodybag, after spending about 2 weeks talking to them, they reluctantly refunded me after accusing me of lying because they had 'no record' of the purchase. Other times, I would try to purchase one and there would be some kind of error and it wouldn't work.
The thing which annoyed me the most and made me leave was the fact that they disabled my Internet due to 'excessive usage and tethering' - I wasn't tethering (I couldn't even get it on my phone if I wanted it) and I'd only used 200MB for the whole month, at the time the £10 Goodybag gave me 'unlimited Internet' - obviously not. I contacted them asking if they could enable it again, they said yes but they'd permanently end my account if it happened again because I'd 'breached their contract' twice. After asking them how, they told me that I'd used excessive amounts in previous months and I'd been warned about it twice, which is absolute rubbish, I'd received nothing from them and I looked in my phone's info, and I'd only used 1.5GB in about 5-6 months, that's probably less than average, not 'excessive'. I told them that I hadn't received any warnings, nor had I used excessive amounts and they simply replied 'are you calling us liars?' - I switched to Three about a week later.