Kaku does not have a good reputation in the scientific community, especially from peers I personally know and who are great physicists. He is seen as a trained physicist with a successful career, but not as one seminal leaders of theoretical physics. He also seems out of touch with particle physics today.
I have however never really taken the time to go personally deep into his works. From the comments of my friends, I will not do it... There are too many more interesting things to do and not enough time, which means that a selection must be made :p .

One important point to keep in mind. Kaku has a theory, and a theory is after all, a theory. Not THE theory. There are other theories on the market. The one that one prefers (and that is not refuted by data) is a matter of subjective choice.
For information, I have myself worked on several of them.

Cheers,

Benjamin

PS: I am chocked a bit by the title of your post. The words Well known scientist...