He is overrated, that's not a criticism as he is good but he's not one of the all time greats of the genre in my opinion. Also it's not fair to compare Em to Kanye, one is a producer/rapper and the other is a spitter so it's like apples and oranges, besides as a rapper he's not as good as Eminem anyway.
Em is not one of my favourites to be honest but he is one of the great rhymers and spitters, you can't really deny it. His way of putting words together and his rhyming dexterity is up there with the best. If he'd have done an album with quality producers when he was at his peak then he'd probably have made a classic and would rightly be regarded as one of the all time greats, but he didn't. Listening to MMLP in 2013, the beats have aged horribly and once you get past the silliness, gimmicks and shock value there are only really 3 or 4 really good tracks on that CD. Slim Shady LP as well but less so, great CD with some of his best tracks but not a classic either in my opinion.