I actually stayed in Uguisuidani a few years ago, intrigued by the name ("Nightingale Valley") and the cheapness of the accommodations.

As others have said, you will notice that most of the passersby on the streets are men, and there are racy lingerie and sex toy shops around. The lobby of the hotel had a sign in Japanese saying that people engaged in the sex industry were not allowed to rent rooms.

However, the neighborhood didn't look or feel at all unsafe, and the hotel was just fine. The only disadvantage was a shortage of places to eat other than McDonald's. The eateries looked as if they catered to the late-night post-revelry snack crowd.

No nightingales, though, or birds of any sort, really.