I would disagree with the first part, agree with the second. I don't think he's considered a disappointment at all. He's a solid rotation player for one of the best teams in the league. Drafts usually don't go 12 deep with stars. If you go through the 2003 draft I doubt you find 12 better players than Nick Collison. Here's my shot at it:
Obvious picks: Lebron, Melo, Wade, Bosh. Probably David West.
Same rough group: Luke Ridnour, Boris Diaw, Kirk Hinrich, Mo Williams, Kendrick Perkins, Chris Kaman Josh Howard (when he was good, bu t he's out of the league now)
Collison is probably one of the 8-10 best players in the draft.
Another way to look at it, past #12 picks.
2008: Jason Thompson
2007: Thaddeus Young
2006: Hilton Armstrong
2005: Yaraslov Koravev
2004: Robert Swift
2002: Melvin Ely
2001: Vlad Radmanovic
2000: Etan Thomas
1999:Aleksandar Radojević
1998: Michael Doleac
That's the 5 years before and the 5 years after. The most recent 2 are the only ones in Collison's league as a player.