I've been a Tesco Mobile pay as you go customer for 18 months or thereabouts - can't fault the pricing and the offers. I am a very light phone user and find that 15 euro credit can last 2 months or more, because of the additional credit I get (When I top up by 15 euro, I get unlimited texts for 30 days).
There have been problems with network reliability - the network went down 3 times in 2 weeks a couple of months ago, and was off air for many hours at a time. If you're on facebook, check out the Tesco Mobile page to see some of the reports and comments.