I actually agree with Redman...and I think lyrically Eminem is untouchable in his area (being a nasty bastard, basically).
He could never consistently use his lyrics to project the world he came from effectively, nor does he really say a lot of insightful things which really make you think (he does both on occasion, but...), which I think is something Nas, 'Pac and Biggie were always good at...but lyrically, he is very good at putting some funny, nasty things onto record.
I don't think we will see another musician like him.