Amukamara is the second ranked corner in the draft and rightfully so. He has good size and strength at 6'1 and 205 lbs. His size gives him the ability to be a solid tackler. He's also good in both man-to-man and zone coverage.
His 2010 stats indicated that no quarterback wanted a part of this guy. He had 36 solo tackles and 13 pass breakups. Amukamara also moves his hips well which means he can change direction quickly and he can close on the ball fast and even when he's beat deep, he's usually in a position to make a play on the ball.
Amukamara has problems defending bigger receivers as evidenced by his game against Oklahoma State. He also has a problem shedding blocks in the running game (which was evidenced in several games).
Amukamara also lacks straight line speed down the field, which gets him in trouble sometimes because he tends to jump on first moves and gets beat on double moves down the field, he also didn't have a single interception last year, and while that may be because no one threw at him, that won't happen in the NFL until he makes QB's pay by creating turnovers.
Amukamara is one of the best corners in this draft and should be available for Dallas. His negatives for the most part are coachable, so he should able to adapt to the NFL game quickly and with the Cowboys secondary, he won't be counted on to save them, but instead improve them for the first season.
Here's some tape of his game in 2009 against Baylor, where you can see his strengths and weaknesses