the marines are the hard corps, you join them if you are trying to test how badass you are.
the air force and navy are easy compared to the marines, unless you're a navy medical corpsman, in which case you could be assigned to a marine unit as a medic.
the army is (and I hate to say it) kind of the ass end of the american military. you have to work harder than navy or air force, the discipline is more bullshit, and you end up getting shot at as much as marines do. but it also all comes down to your Military Occupational Specialty (MOS)...if you're an 11B, you're infantry cannon fodder, but if you test high enough you can go into one of the tech fields and actually learn something useful. My dad was artillery, firing 155mm self propelled howitzers...he's half deaf from all that stuff. but he's the successful example of what the army can do for you; he spent 26 years in, retired as a Colonel (O-6), and gets like 70% of his base pay as his pension. so he wasn't even 50 yet when he got out, so he went to law school.
good things about the military:
uniforms, so you don't have to worry about your wardrobe
mess halls - maybe crappy, but if you spend all your money, you still can eat
potential for early retirement
travel
GI college bill
garrison troops fuck like rabbits
bad things:
war
people trying to stab/shoot/disembowel you
training accidents (I saw a safety manual once with pics; one guy had an unsecured gun barrel fall on his head and it crimped his helmet onto his skull like a fucking bottlecap