My wife and I recently vacationed in Spain for two weeks. During that time I wanted to use my Google Nexus One for data (e-mail, Facebook, language translation, maps), texting, and the odd phone call.
While on a walk through Barcelona, my wife and I came across an Orange retail store. We went inside to see what they offered in a pay-as-you-go format and what we walked out with was their Dolphin sim card loaded up with 10 Euros credit.
It doesn't sound like much, but 10 Euros got us unlimited internet (speed throttled above 500MB) for the whole trip and a handful of calls and texts.
Their coverage was great. I was never without reception. Also, the customer service was fantastic!
Total cost for 2 weeks of mobile telephone and internet: 15 Euros