The quick summary:
He is big, quick and really strong with long arms but has very little experience because he was a basketball player in college. He had never played OT until he came to the Cowboys.
He played reasonably well in 2012 with 1 start at LT and 3 quarters plus overtime in another at LT. He then split time with Free at RT for the final 4 games that season. He was as good as Free.
In the 2013 training he was inline to compete with Free but got injured and missed all but 1 preseason game and couldn't challenge Free.
This past presesason he was reported to have problems, but in the actual preseason games he played reasonably well. He had many dominant plays in the run game and was generally good in Pass Pro, but will generally have 1 or 2 plays bad plays per game that are usually mental.
IMO, the only way a player like this can take the next step is to play in real games. Zack Martin played 52 games on OL in college; whereas Parnell played zero.
When he has had bad plays it usually on his first or second snap of the game. He has been better when he played OT than when he has played as a blocking TE.
I'm convinced that physically he is a better player than Free, but he has much much less experience and will likely make a couple of mistakes if he starts.
