Spain has a big tradition of competition bikers (in the eighties and ninenties the 'Tour de France' price was several times won by Spanish runners). However the Spanish themselves are only recently starting to see bicycle as a way of enjoyment and travel. There are almost no cities with a bicycle way (Barcelona has only started to build it recently) and car drivers are quite unaware and unconcerned about the danger they pose to bikers on the roads.
However one can barely think of another European contry where cycling could be more fun. Good temperature, few winds, sunny, beatiful and diverse nature,... We suggest that if you are considering cycling in Spain you make an extreme use of security measures, keep away from main roads and expect anything. Make sure you understand the current norms that regulate the use of bicyces in the public road network.
Theft is a common thing when it comes to bicycles. Make sure to have a good lock and lock it always to a fixed post. We also recommend you to store your bike in a secure place at night. For common city use its possibly better just to own an old unattractive bike.