Thinking back of the QBs taken very early, there's probably more that have been complete busts than those that have been successful. Seems like the ones that ARE going to do well, it was pretty clear they were franchise QBs. Peyton Manning, I don't think there was ever really any doubt he was going to be successful. Eli's been a little more hit-or-miss in his career, but he earned a ring and can't be considered a bust.
You think of guys that HAVEN'T washed out of the league, they've gotten into high-quality organizations; the Mannings to the Colts and Giants, I think of Drew Bledsoe playing for Bill Parcells in New England, McNabb going to play in Philly, McNair going to Houston. You look at most of the guys who've turned to busts - David Carr, Akili Smith, Alex Smith, etc etc etc, they've gone into organizations in turmoil, they've gotten shellshocked because they had horrible pass protection, they change coaches and co-ordinators every season.
Is it worthwhile to draft a QB first overall when you've got a bad offensive line, a lot of pressure on the kid to play before he's ready, and an unproven, unestablished coaching staff?
Just reading about the kid... the scouting reports read a lot more like the ones you read about the second group than about the first.
If I'm the Lions, I'm thinking about either a stud offensive lineman to protect whomever I draft NEXT year to play QB, or a guy like Curry that seems to be one of the surest things in the draft.