Approximately 3 months ago my 18 month contract expired with O2. Initially I was planning on moving to Tesco Sim Only and get an iPhone 5 when it was released during summer but the iPhone wasn't announced and the Samsung Galaxy S2 won me over along with a previously (for myself) unheard of network called GiffGaff.
GiffGaff is a mobile virtual network operator running on the O2 network, just like Tesco Mobile, but deviates from convention by allowing its users to run certain aspects of the business in favour of payback.
Obviously the payback aspect is one of the major selling points of GiffGaff, but due to the community run aspect the running costs of the company are significantly reduced which means the cost savings are also passed onto the customer.
You can buy credit in 2 ways, a normal top up with cheap tariffs of 8p/min to UK mobiles and landlines, 4p texts and 20p a day (up to 20MB) mobile internet. Alternatively you can buy a goody bag for £5, £10, £15 etc. I personally use the £10 goodybag which gives me 250 minutes, unlimited texts, and truly unlimited web per month.
This is significantly cheaper than the big companies, and while some virtual operators offer excellent deals, many cant compete with GiffGaffs excellent reputation.
In the 3 months I have been with them there have been a couple of little issues with data speed but these seem resolved now, and I can see myself staying with them for quite a while, and more importantly I cant see myself ever going back to a contract.