Well I've seen him for Holland and for Ajax. He's got no stand out qualities. And he's not really a utility player either, not in a top league. He has played at CB, but he's poor there in the Netherlands, so he'd be a disaster there in the Premier League. He's played as a wing back, but he's far too slow to play there in the Prem.
Even in CDM I don't think he has the control to handle the high pressing high tempo style. Since he's not good enough to play LWB or CB in the Prem, we'd be signing him as cover for Carrick as a holding midfielder. But if we're going to sign a holding midfielder De Jong is much much better, Prem proven and probably half the price.
The proof is in the pudding. If LVG thought he was good enough for us, he'd have signed him a month ago. It's a very, very simple deal to make. So we can safely say that he's, at best, a back up option.
I just don't see the point in signing players who wouldn't get near our first eleven, arguably even our bench, with everyone fit.