He's over-rated a little I think. The talk of him being the best defender in the league is over the top but he's a good Championship defender.
As has been said his strengths are in his commitment. He'll throw himself at everything, which makes him very popular with fans but it can also be his undoing at times.
He was culpable for a few goals last year (Reading at home springs to mind) where he charged out of position to make a block, leaving space for a clever opponent to pass into.
At the moment, I think Turner is the better defender but Keogh isn't too far behind. If he works on his positioning and concentration he can play at the top level, but I think Cardiff would be about the right standard of team for him.
If he does go, I'd wish him all the best. I got a bit frustrated with his lack of positional sense at times, but he always gave his all and was a good player for us.