
I'm on Tesco Pay as You Go and have been for years. There are only one or two places I struggle to get a signal, but these are known 'blackspots' for mobiles anyway.
Tesco have had an offer running for ages, when you top up £10, you receive an extra £20 credit free!!! Needless to say, I don't need to top up my phone very often :) I did look at a cheap monthly contract, but to be honest, when £10 of credit can last me for 10-12 weeks, even a cheap deal of £10 a month seems excessive :)
I used to have a great bog standard phone, but it broke last year, so I bought a 'Huawei' smartphone for about £50 (from Tesco, I had a £10 voucher, £10 off the phone at the time, and then £10 of free credit, so it only really cost £20!) and just swapped my SIM card over. It does all of the 'internetty' things that new phones do, but I only use it to go on the internet if I'm in a free WiFi area, otherwise it does eat up my credit.
I can't fault Tesco Mobile for their signal at all, and one the odd occasions I've had to speak to someone, their customer service seemed pretty good too.
Hope that helps
Claire