Technically speaking, Em is near untouchable. When he wants to, his rhymeschemes and multi-syllable rhyme patterns are top tier. Creatively though, I think he's hit a glass ceiling.
When he first came out with the Slim Shady LP, he had off-the-wall never been done before concepts (accidentally killing a girl at a rave by offering her too many shrooms on 'My Fault'... wtfff), clever word-play (Slim Shady, hotter than a set of twin babies, in a Mercedes Benz with the windows up when the temp goes up to the mid 80s) and relevant pop cultural satire (All I see is sissies in magazines smilin', whatever happened to wilin' out and being violent?). IMO, '99 to '02, Em was untouchable and rightfully so (not over rated at all).
Now though, he seems to just be rehashing old concepts (That song with Rhianna is like a watered down 'Kim'

irrelevant name-dropping (mentioning Miley Cyrus in songs for no real reason but just because she's the new trend), and just making borderline corny stuff (So Seductive on his new album was wack as fu*ck). He can still rap as well as ever, but the problem isn't that he's a bad rapper, it's that he has nothing to rap about.
It's like Leonardo DaVinci painting the most detailed pictures of feces over and over again.
And what sucks is that people are buying into it... so he's just gonna be doing the same thing over and over again, cuz apparently, it sells.