Traveliing Spain by train? Tips for Spain?

Has anyone taken the overnight Renfe train within Spain?? There seems to be mixed reviews about this service, but it seems to be the cheapest option and it means I also save one nights accomodation..
I'm a budget traveller so, is it really that bad as some lonely planeteers say?

Also, any insider tips for Madrid, Barcelona, Granada or Seville are appreciated!