I don't really think he's boom or bust. I think he actually has a high floor as a mid-level game manager QB much like Purdy. The one thing that could hurt that is if he can't learn to step up in the pocket and get the ball out before sacks, but both those things seem very coachable.
He hasn't shown to be a guy that can take over games which is why it's hard to see him as a high first round pick because his ceiling doesn't appear to be very high, but I also think he doesn't have a lot of risk. He really seems like a 2nd round guy, but QBs often get taken a round earlier than they should.
BTW, Kiper released a mock draft today in which he has us taking Dart at #9 (Ward and Sanders go top 3). I would not be happy with Dart at #9.