Pros
Build a good work ethic
Understand responsibility and what it means to be a man, and have honor
Experience true adversity for the first time in your life
Start a college education, or get money for one
Support your family if you have one
Make friends for a lifetime
Cons
Might die (if you're combat arms, otherwise don't worry)
Might lose life-long friends
Find out you suck at the army, go AWOL and become felon
You're marriage might fall apart if you two aren't 110% committed.
Pretty simple really. I used to think the army and marine commercials about "be all that you all can be" were BS, but now I believe it, truly.