^ With Kiss Kiss Bang Bang under his belt, not to mention screenwriting duties on Lethal Weapon, The Long Kiss Goodnight, The Last Boy Scout etc, Black has a pretty good track record, certainly comparable to Favreau's circa IM1..
I don't know if you could call that proven and if he was he wouldn't necessarily be cheap.
Fortunately Marvel has been really lucky lately with their directors unlike DC. Don't get me wrong i'm still hopeful IM3 will be great and with Black maybe they are going to explore Tony's alcoholism in the next movie. Still, I really wanted to see where Favreau was going to take the character especially Tony's new long term relationship with Pepper.
I'd say Black's pretty much on the same level as Joss Whedon, who they nabbed for The Avengers. Respected writer with one acclaimed but not world-beating directing credit under his belt.