Howard Stern and Britney Spears Credit: Dario Cantatore/Getty Images; Steve Granitz/

Not everyone is thrilled about Britney Spears officially signing on for The X Factor.

At a press conference in New York City Thursday, America's Got Talent judge Howard Stern, 58, joked that Spears' hiring is "a wonderful decision" because she "still thinks the earth is flat."

"I think we're going to tune in to see her, to see if she can function through the thing," Stern explained. "As far as any real criticism, I think Simon Cowell and L.A. Reid will do that. . .I think Britney will sit there and eat a lollipop and wear a sexy outfit.

Said Stern: "I'll tune in to see what kind of train wreck she is, absolutely."

Spears will get a reported $16 million for doing the FOX show. "There's been a lot of back and forth over the past few weeks as they negotiated the small details, but she's on," a source told E! News Wednesday. "It's all completed and Britney is beyond excited."

The pop star -- engaged to Jason Trawick, 40 -- "can't wait to sit down next to Simon at the judges' table and give this everything she's got," the source continued. "She is excited about showing the world that she can do this, and do this well."