"Japan is good at copying American technologies and using them to create better products than its American counterparts, like cars and electronics" - so I have been told about Japan's economic success. I'd say that the same can be said about Japan's copying American sub-cultures. In particular, I've been overwhelmed by influence and impact that American hip hop culture has made on Japan during the past decade. I really did not know, for example, Def Jam Japan was established until very recently.
I have been really impressed with how R&B has been absorbed by the Japanese music industry and come out as truly unique like Toshi Kubota, Kiyoshi Matsuo and so many others (also see Japanese R&B (J-R&B)), but I did not have good impressions about Japanese Rap. It's primarily because it sounds "funny" when Japanese rappers try to rhyme; Japanese finds its beauty in lyrics through making the number of Japanese alphabets, or phonetic units, limited in each line and not through making rhymes. For example in Haiku, a poet needs to limit his/her use of alphabets (which are like mono-syllables in English) to 5 in the first line, 7 in the second and 5 again in the third. The art of Haiku comes from how the poet can express his/her messages beautifully and phonetically within his/her limited use of alphabets (hence words). So, Japanese are not used to rhyming, which to me is part of what makes rap music a form of art.
Yet, I must admit: Recent years, Japanese rap music has evolved and leveraged itself to a great extent. Japanese rappers still try to rhyme; but, they do choose (seemingly) words carefully so that rhyming doesn't sound really funny (which has been a great discovery to me as a Japanese!). On the top of it, they add a bit of Japanese pop flava. So, they do sound really cool and unique. Dragon Ash and other artists under the Def Jam Japan label are really good. I think they can't be merely labeled as Japanese Hip Hop any more - so how can we call it?