Just gonna point out that Henry, Bergkamp and Vieira were pathetic when we signed them too. How a player currently performs in a particular club or setting does not necessarily determine how he'll perform at a different club or setting.
I agree about Yildiz for what it's worth though, bloke's too slow to ever be a top winger. He needs to move into the middle if he's ever going to fulfill his potential. At left wing he's a lost cause, and with an inflated transfer fee he's the exact sort of gamble you shouldn't make unless you're sure about how to use him.