Pros
-housing and food taken care of
-learn discipline and sense of purpose
-brotherhood
-respected by society
-GI bill
-get to travel
-automatic connections that you can use later in your career
-some girls have military fetish
Cons
-somebody constantly telling you what to do
-can't have your own style
-could die or become seriously injured
-if you do live, the experiences will scar you for life
-forget having a wife. If you get married, she will absolutely cheat on you. Many women marry military guys strictly for the benefits
-if you have kids, you could be away from them during crucial development periods (they'll never be young again).
-you could have strong moral disagreements with what you're being asked to do (such as the war in Iraq).
Personally, I couldn't deal with not having control over my own life. But there are people that have life changing experiences in the military. I've heard the army is the worst and least prestigious division to go into (being on the front line). The airforce, navy, and coast guard are much easier (but have stricter entry requirements). Marines and Navy Seals are the hardest but also the most prestigious.