I used Yoigo as my provider for about a year.. this was about a year ago (I had a contract).
I was satisfied with them. They rent capacity from Movistar (Telefonica), so coverage is good. The only problem I had was that sometimes my sms messages that went out of Spain (back home to Norway) mysteriously disapeared. I sent it, but it was never recieved. Not sure if this was a handset issue my my phone or the one in Norway, but it happened a few times... not sure...
Other than that, go for it. They have good prices. And by the way, I think you can add more euros to the account at many post offices and gorecy stores (at least the larger chain stores).
Buen camino!
Ivar