At this point you have to say he's the best player from that draft, but the NFL Draft is such a crapshoot and I don't think you can say with certainty that the Texans made a mistake until next year or later.
And recall that last year before the draft, Reggie Bush was the no-brainer #1. Vince was only discussed in passing, and it usually involved where he would be picked relative to Leinert. I don't remember any serious discussions about Young going #1 until later in the season when his good start (and the final confirmation that Carr sucked) lead people to believe the Texans made a mistake.
Anyway, I don't mean to imply Vince Young is a disappointment. I just think the NFL Draft is such a crapshoot sometimes that it takes an incredibly special player to be able to say ANY team, regardless of need, should take that player with the number one overall pick. And again, in May, 2006... that player was Bush, not Young.