He's played well there whenever he's played for Ireland. From what I've seen of him (and I've watched a fair bit of him) he's best as a central defender. It's obvious he isn't a natural full back or defensive midfielder.
I don't think he's a particularly valuable member of the squad. He's only a player who comes in for the last part of the match or who plays in a game that Man Utd should win easily to give other players a rest. I understand that it's difficult to leave such a successful club, especially when he's on a decent wage, but the fact is that he would have become a much better player had he moved on a few years ago, and may have become so good that one of the "big 4" would have signed him later on. I think staying at Man Utd has been detrimental to his development. I don't see how anyone could argue that point. Who knows, maybe he'll have an Indian summer and become an excellent player like Dunne, but the chances of this are unlikely unless he leaves, which I doubt he will.
He's started 6 league games out of 26. I don't think that's a great percentage. He's played 673 minutes out of a possible 2340 in the league. That's less than 30%. No player his age should be happy with playing that little game time. Don't get me wrong, I actually like John O'Shea, but I just think he could have done a lot better for himself by leaving Man Utd. I just think that it's down to laziness and fear of leaving a relatively cushy life behind that has prevented him from leaving.