I'm not an American living in Japan - I'm Japanese living in America, but I've seen enough of the former to say the answer depends on the person.
Some Americans are going to have a great time while some are going to regret their stay.
Knowing both cultures pretty well, My word of advice to an American going to Japan is:
1) Learn as much as possible about manners and etiquette. Manners are important in Japan and are different from U.S.
2) Learn to read the atmosphere and brush up on your social graces. It's impossible to read and remember every dos and don'ts from a book, so good people skills helps smooth things over.
3) Japan is a group oriented country rather than an individual oriented country. Make good presentable Japanese friends, specially of the same sex as yourself. From everything to getting a job or finding an apartment, not only can they help you by word of mouth, but they give you credibility.
These three things should help you the most into integrating into Japanese society and start opening up doors for you.
Final not: If your going to Japan for the girls, then leave the average girl alone. you'll just add to the bad foreigner gossip and hardly score anyway. The Japanese girls that want foreign guys go out of there way to seek them out; military bars, sports bars etc... where foreign guys hang out, that's' where they go. The ones I saw at the Nagasaki Navel base bar were very pretty but they didn't want to have anything to do with me since I'm Japanese.
