It seems like, even in the first round, there's about a 50/50 chance of QBs of turning out good-great...
If we look at recent drafts when more than 1 QB was picked in Round 1:
2003:
Carson Palmer - so far, decent
Byron Leftwich - should be good, at least
Kyle Boller - not shown much
Rex Grossman - who knows
2002:
David Carr - Could be good
Joey Harrington - Incomplete
Patrick Ramsey - Ok.
1999:
Tim Couch - Out Of Football
Donovan McNabb - Great
Akili Smith - OOF
Daunte Culpepper - Great
Cade McNown - OOF
1995:
Steve McNair - Great
Kerry Collins - Good
Even back to 1983:
John Elway - Great
Todd Blackledge - Bust
Jim Kelly - Great
Tony Eason - Decent
Ken O'Brien - Decent
Dan Marino - Great
You may quibble with some of my individual assessments, but the fact remains...
it's a bit of a crapshoot...