Where hip-hop enthusiasts who don't mind some more extravagance will go for the good shit. Not necessarily limited to Japanese lyrics(in fact, quite a bit of J-Rap artists out there will feature English-speaking lyricists in their songs if not a remix), Japanese hip-hop is more focused on pleasant, jazzy, and exotic beats, usually involving the piano or other instruments.
It is very different from US hip-hop(even from a lot of underground artists), but there's a certain beauty to it that may make it almost as(if not, even more) appealing than the typical underground artist. It's obviously an acquired taste, so it may not appeal to everyone.
If you want to get into Japanese hip-hop, start with the likes of Nujabes. From there, you can branch into artists like Nomak, Shing02, Shin-Ski, Martiangang, DJ Mitsu the Beats, etc.
Person A: Hey, what are you listening to?
Person B: Nujabes - High 2 Lows ft. Cise Starr. Dope track.
Person A: Whoa...this sounds way different than what I normally listen to. I don't see much hip-hop out there with a piano flow like this! I'm feeling this!
Person B: You just got a taste of Japanese hip-hop.