Offensive Tackle is the most consistent pick in the draft meaning the lowest probability of bust in round 1. Sure there are busts but far fewer than any other position.
MUCH MORE IMPORTANTLY; Winston Justice is a beast. He can come in and start form day 1. He WILL be a pro bowler and will anchor our line for the next 10 years. Best thing is he is VERY versatile and can play either tackle position. That is very important in an O-linemen because in case of injury you can maximize the other player's ability since they don't have to play a specific position. For example if the coaching staff feels that Carey is better at LT than RT, there is no problem moving him to the left side because Justice can play the right. He would be able to play at a pro bowl level at almost any O-line position (excluding center). Obviously it would be best to have him at one of the tackle spots.
At USC he played RT because Leinart is a lefty so his blind side is the right. I'm not too worried about the character issues and hopefully those will allow him to slip to us. However I wouldn't be opposed to trading up for him.