There's plenty of good food here and things to do, but I wouldn't recommend more than 2-3 days. It's a grittier and dirtier city than Tokyo with more drunks and homeless people, it seems to me. There's also plenty of yakuza.
For those seeking the Amsterdam-like experience, and I'm not talking about Mary Jane laced cookies, there's that too.
I'll be back in Tokyo this Friday.