Here's what Mocking The Draft says about the pick:
It really wasn't a surprise to see the Vikings use this pick on a quarterback. It was a shock to see it be Stanzi. Although I'm not as high on Nevada's Colin Kaepernick as others, he's a much better player than Stanzi. It sort of seems like people are touting Stanzi as maybe the next Tom Brady solely to take a flier.
I like Stanzi a lot as a fourth round pick but not this high. Alas, that's life when you don't have a future at quarterback -- you have to reach.
For the full explanation of the pick, check out Mocking The Draft.