In a strange way, I feel he didn't fulfill his potential.
When he first broke through in 2002/03, he was this marauding full-back who nutmegged Figo or drove through the Newcastle defence and smashed the bar, leading to Giggs' tap-in in the 6-2 win.
But he seemed to go downhill after that, ending up being this lumbering, clumsy oaf in defence, like when he gave away that penalty against West Brom at home in 2004/05, the game where Kuszczak had a blinder for them and we probably decided to sign him. At least he did score that cracker at Highbury in that season!
He improved as a tidy central midfielder towards the end of 2005/06 alongside Giggs and played a valuable role as a squad player popping up with important goals in 2006/07.
2008/09 was probably his second best season after his breakthrough. He was helped by Wes Brown's injury problems and managed to nailed down the RB spot, after Gary Neville was clearly past it and Rafael was still developing. He scored an important goal against Arsenal in the CL and was probably our best player in the final against Barca.
But still, I feel like we missed out on that wiry, surprisingly quick and adventurous full-back O'Shea was when he first broke through. He ended up becoming pretty much the opposite.
A solid, but not particularly quick utility player who filled in wherever possible, even in goal! He also popped up with some memorable and often important goals for us.