Favre + Fantasy = Fail
Don't do it.
I could just end this right there. You shouldn't need me or anyone else to tell you not to do what I know you are thinking of doing. My backup quarterback is Kyle Orton, and I didn't come close to thinking of picking up Favre. He is going to take up precious room on your bench, and they payout will never match the amount of attention the name is getting.
First off, let me clarify that I am writing this with just a hint of bias. I hate Brett Favre. I would rather have Chad Ochocinco team up with John Madden to commentate every football game to the end of time, then put up with Favre anymore. He has ruined everything about the NFL and the upstanding Hall of Famers who actually retired the right away; by going through with it. So that being said, I still believe Favre would be a horrible fantasy selection.
1. Inconsistent targets: Bernard Berrien is going to make highlights, not a consistent target. The rest of the Vikings receiving staff is unproven and will not be able to adapt to Favre's gunslinger style in the short amount of time they have, thanks to Favre putting himself above player status and skipping training camp.
2. Heavy Run Offense: This team is built around AP. He more than earns the nickname "All Day". Favre will be on the field because of his brain and ability to manage a game, not for the stats he is promising. Adrian Peterson will put up every point the Vikings score this year (almost) and Brett Favre will be left with 200 yard, 1 TD, 2 INT -type games.
3. Shouldering the load: I talked to some of the Archeologists at the Smithsonian museum. They said that fossils in Favres condition need to be preserved in a very stable, and non-active environment in order to obtain substance. Favre's shoulder will not last the season. If he is still playing by week 17, than the pain he is feeling in his shoulder will be causing him precious inches of accuracy on his daring throws. One doesn't become the NFL's all time leader in interceptions by playing conservatively.
So if you are needing a backup, and you think Favre being available is just too good to be true, well, trust me...it is. Pickup someone with a better chance of an upside to get you through your #1's off week or bum throwing arm. Other than that, just sit back and enjoy the self-destructing fireworks that is Brett Favre's career.