Lots of research has been done into the effects of bilingualism. The general view is that it has many benefits, it depends however on the way you are a bilingualist.
The Economist writes that bilingualism contributes to delaying dementia, having an edge in executive control by helping to perform complex tasks by focusing attention, and updating working memory. So far novel research is unclear about how and when you would have these benefits. It all seems to depend on how the languages you speak are related and how you are using them. Switching regularly is a big factor in supporting executive control.
Nevertheless, aside from the subtle benefits that are scientifically researched, being bilingual gives you the opportunity to expand your cultural horizon. Reading in different languages gives you more perspectives and create a deeper understanding of other people.
One fun way to boost your learning of a second (or third, or fourth) language is interlinear learning. It is especially gratifying if you have a basic knowledge of another language. Interlinear reading boosts your vocabulary while you have fun reading.
What is not to love about that! Check out Hyplern.com for hundreds of stories from French to Russian to Galician. And Chinese is to come out soon.