I think listening to eminem is a great way to improve your english. His lyrics are brillantly constructed, he is definitely a lyrical genious in that his play with and on words can be mind blowing at times even for native speakers, and his much is just straight powerful. I think listening to songs of his over and over, and looking up words you aren´t familiar with in the dictionary is a great way to expand your vocab. Of course, sometimes he chooses words that aren´t widely used, but all in all, I think it´s a really positive way to practice your English. I do the same with Calle 13 and Spanish.