Probably below average to average. I say this because of the following,
1. The Vikings offensive line is horrible. Out of the whole thing, only Steve Hutchinson is a NFL starting level player. McNabb is going to be hit constantly as McKinnie and Loadholt stand there looking around. The lack of success from Vikings QBs last year was not all their fault. The line allowed 36 sacks, which was middle of the league, and 87 hits which was ninth worst. Only maybe 8 or 9 teams have a worse line then the Vikings.
2. Have you checked the wide receiver depth chart? Outside of Harvin, what is there? The next two best receiving threats are Shiancoe and rookie Kyle Rudolph. Berrian, who looks like the other side right now, is practically worthless and no better then a 3 on a bad team, and come on Igelsias and Camarillo? The upside is that McNabb performed well for years in Philly with groups of nobody receivers and a decent tight end or two. The difference is that Westbrook was a lot more dynamic then Peterson, and McNabb has a few more years on the tires.