My brother was a machine gunner (and also did a stint of Intel) in the Marine Corps and I also dated and was engaged to a Marine at one point in time. One thing is absolutely sure about the military, like all things it includes both amazing individuals of high quality character and miserable scumbags. My brother is definitely one of the former, very hard working, handles his leadership with humility, treats people with respect and lives his life with integrity. I definitely have encountered the other kind as well though, in fact my brother even used to warn me about them. Overly cocky, big talking guys with pretty much no actual self respect who use the military to feel macho. Usually these idiots are the least capable and the least seasoned, though they like to think otherwise.
The Marine Corps was a very refining process for my brother, his experience is similar to what @kyuuei described as far as developing his work ethic, sense of self, leadership and chiseling away some of his negative habits. I think it would be good for a lot of people to go through a boot camp scenario (though Marine corps boot camp is kind of a different animal than other branches-- there's a history channel special on it which is awesome).
I'm very proud of my brother and he's never regretted his time in the service, but he's also very much embraced the freedom from it now that he's out. It gave him a lot of oppertunity for growth while in, and then paid for his education once he was out. I also think it helped him determine the kind of man he wanted to be because of the high quality of leadership he had-- well certain individuals anyway, haha.