Just transferred from Vodafone PAYG to GiffGaff myself. When I rang for the PAC from Vodafone, they did ask why I wanted to leave and I said GiffGaff unlimited data on my iPhone and the guy pretty much said 'fair enough'! One thing, double check your phone is definitely unlocked before transferring your number to avoid having to buy a £15 O2 handset to use while you get your phone unlocked 
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I was spending £15-£30 on Vodafone for packs and topups a month. I was also tired of their terrible website, it seems to be a modern flash driven site but built on every previous version. It's SO Slow and clunky. It looks modern and up to date but you click lesser used links and you find bits in old formats and fonts, or it just doesn't work. Also there's no way I found to get Vodafone to tell you how much data allowance you have left, like they do with the minutes allowance. Their FAQ suggests using an app to do it, which is ok but, awkward and annoying especially if you have packs starting on different days. They must have the information to be able to text you when its about to run out and then charge you the daily MB rate, but they choose not to tell the customer, which is just rubbish.
.I was spending £15-£30 on Vodafone for packs and topups a month. I was also tired of their terrible website, it seems to be a modern flash driven site but built on every previous version. It's SO Slow and clunky. It looks modern and up to date but you click lesser used links and you find bits in old formats and fonts, or it just doesn't work. Also there's no way I found to get Vodafone to tell you how much data allowance you have left, like they do with the minutes allowance. Their FAQ suggests using an app to do it, which is ok but, awkward and annoying especially if you have packs starting on different days. They must have the information to be able to text you when its about to run out and then charge you the daily MB rate, but they choose not to tell the customer, which is just rubbish.
