Malcolm Gladwell famously argued in 2008 that the performance of quarterbacks in the NFL "can't be predicted:"
This is the quarterback problem. There are certain jobs where almost nothing you can learn about candidates before they start predicts how they'll do once they're hired. ... The problem with picking quarterbacks is that [U. of Missouri quarterback] Chase Daniel's performance can't be predicted. The job he's being groomed for is so particular and specialized that there is no way to know who will succeed at it and who won't. In fact, Berri and Simmons found no connection between where a quarterback was taken in the draft—that is, how highly he was rated on the basis of his college performance—and how well he played in the pros.
Chase Daniel, who finished fourth in the Heisman Trophy voting in 2007, went undrafted by the NFL. Yet, he's now in his fifth year in the NFL, and has a spectacular career completion percentage of 77.8%. Unfortunately, he's only been allowed to throw 9 passes over five years.
There are many examples of bad quarterback draft picks, such as the wasting of the #2 draft pick in 1998 on Ryan Leaf.
Still, although I haven't been following football closely this year, I've gathered the impression that the #1 pick of 1998 remains gainfully employed in a quarterbacking capacity.