I think it is a teeter totter based on how Favre plays.
Favre plays poorly: Packer fans gett the better as Vikings get the worse. not only do we get to say "Na-Na we told you so" to Favre, but we can rub it in even more to Viking fans.
Favre plays well: it is better for the Vikings and worse for the Packers. this is because not only did we turn our nose at him in in the end, but he still had it in him. Especially if he takes them deep or all the way.
But there is a caveat. Let's say that the Vikings do well, no matter how well (even say they get to and win the Superbowl) We can always claim that it took a Packer Legend to get them to the promised land!
They would never be able to live it down.
I also think that to not be worried even if they get Brett, is a mistake. A big one. Make no mistake, Favre can play. Favre can win. Favre is a much better QB than either of their choices at the moment. He greatly improves a team that is, what Favre has, away from having all the pieces.
He will bring the "We can pull out a close game and I will lead you there" attitude they have not had since Fran Tarkenton. That nshould be good for 2-3 wins at least this next year.