Damn,
I've lived in Japan for about 5 years, two different times. Once in the 90's and also in 2000's, and I'm about to move back again. This is what I have to say.
1) Ignore the posters who are not currently living in Japan. Japanese culture shifts pretty quickly. The Japan I lived in in the 90's was not the Japan of the 2000's. There are far more foreigners living in Tokyo now, and many of them are from non-1st world countries. The result is Japan is becoming more xenophobic again.
2) Your race. Just like any other place, people have preferences. Some girls like tall guys, some girls like short guys, some girls like black guys. Contrary to many people's beliefs, Japan is not a place where people are just looking for sex.
3) Ease of picking up girls. It's all about you. Can you read the girl in the massageplanete? Can you tell she's interested? Frankly, after I lived in Japan the first time, what I got was a confidence boost that carried over to the states, and I got more play in the US than I ever did in Japan. You can't go to a country and simply think the women there are somehow fundementally different than in your own. That just makes you stupid. If you can be smooth or funny in English, have funny body language, that might work. As in any environment, knowing what kind of girl you attract, you want to attract and HOW to do it, via body language, etc is the real key. If you are fat, overweight, greasy hair and stained t-shirt, weazing when you walk... Then you are not going to get any digits at the massageplanetes. If you are well put together, fluent in Japanese, and walk into Doutour and stare at the girl next to you... you won't get any numbers. If you are dressed okay/appropriate, walk up to a girl at 109, ask in for a light, and ask a girl if she speaks english, then chat with her in fluent Japanese when she doesn't, you might get some. I have.
For the record, Ozgaijin's advice about dressing "nice" is way off the mark. It's all about what's appropriate. Are you going to Womb? Don't wear a suit. Are you going to Shibuya in General? Don't wear a suit, look trendy, wear a band t-shirt, casual, cool. If you are going to Heartland, you are a douchebag. Wear a suit. If you are going to Motown, the pinay girl you are talking to is a prostitute, and/or a man, and you are too drunk to notice. (Just kidding, Motown can be fun). If you go to ni-chome, no one told you its the gay section. If you know, and you still go, you know something that most foreigners don't.
Seriously though. It's not about "JAPAN". It's about you.
The only exception is this: I personally advise against getting married to someone you meet in Japan. I know too many friends who have gone for a ride on the green card divorce train. (Actually, this is not exclusive advice to Japan either... See how that works?)