Pros
Great opportunity to have experiences that others pay to have, although your MOS (job) will dictate most of this. In 4 years I got to see over 14 countries, work with demolitions, ride in many helicopters and amphibious vehicles, and made friends who quickly became family. The discipline and hard work makes any job afterwards seem like a cakewalk.
Cons
The cons are vast on the enlisted side. To some people the Marine Corps may be thier best career choice but I would advise anyone with a better plan to get out after thier 4 years. 1. Your experience will be dictated tremendously by whomever is in charge of you. I got lucky with my leadership but I saw many good Marines whos life was made miserable simply because thier leadership was miserable. The longer you stay in the better chance you have of spending 3 years stuck in a situation like that. 2. The Marine Corps will always come first no matter how hard you try. A good leadership will allow you to leave work to for important personal things but if they want you there for something (often times for what seems like nothing) then you WILL be there. I am yet to be able to spend an anniversary or Valentine's day with my wife. 3. No matter how cool your job is it will quickly become mundane. At first my demolition ranges were the most exciting thing I had ever done. I never thought it could get old to blow things up all day in a field but there I was, sitting in a bunker while time fuse burnt, fantasizing about watching Law and Order with my wife. 4. At first your life will be micromanaged and then you will be expected to micromanage others. Your leadership is expected to advise you on every personal and financial choice you make. After that you will do the same for others and thier poor personal choices will be your responsibility. 5. Retention is at an all time low and good Marines are often times forced out due to a black and white system. I saw good Marines, who served frequently and honorably, not allowed to reenlist due to either technicalities or someone else's mistake. Also right now an NJP is pretty much a career ender. A 20 year old Marine, with 2 combat tours, causing no problems, can get caught drinking in the barracks which would get him an NJP. Now this small personal mistake will foreshadow his tremendous service to his country and the Marine Corps and his career will be haulted at that moment. 6. My biggest and most logical con is that the financial glass ceiling is very low. You will always make enough money to get by but never enough to become something bigger. If you plan to never get married then the money and lifestyle is not bad at all. Anyone who wants something bigger for thier family should look elsewhere. Even if the Marine Corps is your only route to college, and now you are 4 years deep in the Corps and 4 years behind the normal civilian track to success, and it seems like staying in makes the most sense at this point, your potential is still higher by getting out and going to school. If you did 4 years, got out, and went to school for 4 years, once that degree was done your starting salary at your job will probably be lower than the salary you would have had at your 8 year mark had you stayed in but your civilian career will quickly take you to beyond where the Marine Corps will. When you make the rank of SSGT most of your job is clerical anyway. You could be doing the same office work, with less hours and less stress, in the civilian world for more money.