Well...I love 'Ed Wood' but I have to say I think Sam Jackson got robbed in '94...

Short answer: not very, in my opinion. Longer answer: what do you consider success? Or, for that matter, 'professional legitimacy'?
Big Boxoffice returns? The power to 'greenlight' a film? Both of these could be definitions of being a successful professional, depending on your point of view. A select few African-Americans hold this kind of power in the film industry. Something a little more ethereal, like social impact? Harder to gauge but arguably important, and possessed by probably an even smaller circle of creators. More vital to me, what happens we seek to define these concepts in light of American ethnic differences? I believe we would find that we contend with different versions of success and legitimacy designated for specific ethnic populations, with variations applied both by those within the group as well as those outside it.
We can take those difficulties straight to the numbers of the most financially successful members of these groups and the black and whites of the awards shows and become further confused (or vindicated) because there are clearly contradictions. Beyonce and 50 Cent have topped some year-end "Top Earning African Americans" lists and been in some successful flicks but don't hold your breath for either of them polishing a statuette anytime soon. Will Smith has a couple noms (and a fortune) but he made his name in Television and Blockbusters, the anti-Oscar. More to the point perhaps: at the end of the day Spike Lee and Albert and Allen Hughes haven't even been nominated once for an Oscar. Any Oscar.
So what does any of that say about our standards of "success and legitimacy" that we're working on here? I think it leads us to the question of the success and legitimacy of the Oscar. Doubly so of the academy, and thus, of the industry itself. Because that's pretty much all Oscar is, the outward manifestation of the whims, vagaries and prejudices of the majority feelings of the system as it stands, however that may shift from year to year. And as soon as you start to question the industry you can get to the real 'meat and potatoes' that's underneath this argument: why aren't there more and better roles for African-Americans in American Cinema and why are the better authors in American Cinema that happen to be African-American not being recognized.