I applied through a recruiter. The process took 6 months. I interviewed at US Marine Corps (Houston, TX) in Oct 2011
Interview
First the recruiter will talk to you. Ensure that you are honest with him or her. The Marine Corps does not want a liar or cheat... the Marine Corps needs honest and trustworthy men and women to carry out combat operations and support to combat operations against enemies in foreign lands. Simply put, be honest... they will figure it out in the first 5 minutes anyway.
I applied in-person. I interviewed at US Marine Corps (Washington, DC)
Interview
It was a very easy but a lot of paperwork. Interview consisted of medical check, mental health check, and a drug test. There were multiple tests that had to be taken on a computer and a very long background check.
This was a tough interview process and the screening was rigorous. It will take a long time. I recommend getting started early. This is a long term commitment. You need to be ready for it.
I have to do this for some reason to get access to the GD's website. Interview process, walked into the recruiting office and said "sign me up". Recruiter was a friendly guy, said "Ok!".