Well, I've seen very little of him in the Eredivisie, but like Gian said, most analysts seem to think he's the solution for the NT's right-back problem judging from his performances.
I've seen all of his NT games though ( he hasn't been part of our NT for very long, so it's only a few games ) and he does look very good. He's tall, strong and pretty damn fast. He's good with the ball at his feet and not just a brainless runner either, he's often involved in build-up play and usually looks up before he crosses resulting in some clever balls instead of just whipping it into the area like plenty of defenders do. He's had a couple of assists where he picked out exactly the player he wanted ( Netherlands - Estonia WC Qualifier comes to mind ).
I can't however say an awful lot about his defending abilities, because these WC Qualifiers have been one-way traffic 90% of the time. I don't remember him screwing up when being put under pressure though.
All-in-all he seems like a pretty good all-rounder who certainly has his best years ahead of him as he's only 23, but battering some pretty poor sides often makes players look good. You can see that he has the ability to make a good player by the way he handles the ball and his decision making though.
EDIT:
http://www.youtube.com/watch?v=UGVI03SnGJ8 Here's a link to said game, where he's involved in quite a few attacking moves and delivers some very neat crosses/passes.