Spanish or Castilian is an Iberian Romance language, and the second (perhaps third or fourth) most spoken language in the world.
Spanish is spoken by more than 500 million people worldwide and is official language in 21 countries, with over 330 million native speakers. In USA alone there are more than 30 million native Spanish speakers and it is the number is still increasing.
Spanish is an official language in many international organizations (i.e. United Nations, GATT, etc.) and is widely used in the world of art and tourism.
If you can learn Spanish, you'll have a head start in learning the other Latin-based languages such as French and Italian. And it will even help you learn Russian and German, since they too have Indo-European roots and have some characteristics (such as gender and extensive conjugation) that are present in Spanish but not English.
Whether you enjoy talking, reading, or mastering challenges, you'll find all of them in learning Spanish. For many people, there's something inherently enjoyable about successfully speaking in another tongue. Although learning a language can be work, the efforts pay off quickly when you finally get to use your skills.