Career postseason rating;
Favre - 86.3
Steve Young - 85.8
Peyton Manning - 87.6
Tom Brady - 85.5
John Elway - 79.7
Dan Marino - 77.1
Troy Aikman - 88.3
Warren Moon - 84.9
Jim Kelly - 72.3
I'd say you're overstating his playoff failures as well considering he's on par with all of his contemporaries sans Kurt Warner.
.. and Favre is supposed to be merely a great regular season QB, yet his postseason passer rating (86.3) is higher than his regular season passer rating (86.0)
Passer rating has nothing to do really with what's being said. He might play three and a half great quarters but when you absolutely need a play from him more often than not he'll screw up.