Will Marissa Mayer Save Yahoo! or Run it Into the Ground?

This is only her first day on the job as CEO of Yahoo!

, but Marissa Mayer is already being criticized as an unusual hire with "no real leadership experience."

"I have to tell you, I don't know what the Yahoo! board was thinking,"

Herb Greenberg ranted last night. "This is a wonderful opportunity for her but this is also a remarkable challenge for someone that has no real leadership experience." Mayer, a former Google

exec responsible for the

, is an engineer and inventor credited with several patents in artificial intelligence and interface design. Colin Gillis, the senior technology analyst at BGC Partners,

that Mayer will have "about 100 days" to prove that she is right for the job. "She needs to come in and prove herself," he said. "Obviously Marissa is very well-known, but she's still going to be new as a CEO and we're going to have to see where she is going to take it." Trip Chowdhry, the Managing Director of Equity Research at Global Equities Research, has expressed some concerns as well. But his worries have more to do with Yahoo!'s track record than anything else. "This is the fifth CEO at Yahoo in five years," Chowdhry said in e-mail update on the company. "We doubt if fifth is the charm." Chowdhry had previously predicted that Yahoo! shares could

under the guidance of its last chief executive, Scott Thompson. But Thompson -- who came to Yahoo! after serving as the President of PayPal and the Chief Information Officer of Barclays Global Investors, among other prominent roles --

. Thompson's deception led to his immediate resignation. Prior to Thompson's appointment, Yahoo! was led by Carol Bartz, who brought

and

to the company. How could Mayer do any worse? Contrary to her critics, some of Silicon Valley's most powerful people have faith in Mayer's ability to lead Yahoo! to success. Last night, Netscape co-founder Marc Andreessen told

why Mayer is a great choice for Yahoo! "Three reasons: (1) She's a proven manager at scale," he said. "She knows how to run these companies at scale. There aren't that many product managers in our industry who can manage at scale. (2) She's a proven product leader. (3) She knows the Internet inside out." "A lot of people who may have had those first two may not have the third," Andreessen added. Speaking to

, ThinkEquity analyst Ron Josey said that while there is already a "look and feel" across Yahoo!, "I wouldn't be surprised if you see a more unique one across the brand going forward." The Wall Street Journal also interviewed Evercore analyst Ken Sena, who said that Mayer's background "might make her especially suited to make rational decisions as to what should be kept and what should be disposed of." Chowdhry had a few positive things to say as well. "The last four CEOs were clueless and did not understand [the] Internet," he said. "The new CEO understands [the] Internet very well."