Common=Black Thought. Both are criminally slept on, and Common isn't in no way over rated, dude is lyrically one of the better more consistent artists in hip hop. Have you been to a Common live show? There's NOOOOOOOOOO WAYYYYYYYYYYY you'd ever even THINK to say dude is over rated after you see him live, dude lives and breathes hip hop..
Now as far as mainstream goes, he did go under the radar til around 03 like the original topic poster said, however just because he's enjoying the success of his craft as well as getting the movie and tv love, doesnt all of a sudden mean hes over rated, just means he paid his dues...
Black Thought again, is criminally slept on and lyrically is up there with Common, both have stronger catalogues (in my opinion) than Mos Def and Talib Kweli but all four are talented individuals.
Common's in my personal top 5, black thought is top 10-15