Too early to judge Mayer, indeed.
But judging from the past performance of Yahoo's board, I wouldn't be optimistic.
First, the board obviously don't know what to do or where to go, let alone choosing the right person to lead.
Second, I bet the board must have the mentality of "because she was there when Google took flight she must be good". They may be right but at the same time it is an indicator that the board has just throw their hands up in air: "that's it, we don't know what the fuck to do with Yahoo so let's just take a bet"
Third, Mayer won't be able to replicate Google's success. Yet, IMO, to turn around Yahoo, one needs no less than Google's trajectory.
IMO, Yahoo is like Blackberry: they had the capability but totally missed the window of opportunity --> Too late to save.