I don't have a problem with Brett coming back again, he could have handled it better... (ok he could have handled it a lot better), but if he wants to play and has the ability to play i see no problem there.
For the Vikings it's a good move, Tavaris is terrible and Sage is slightly above terrible so it helps them on the field (providing Brett is still able to play at a high level after surgery a few months ago), as well as with ticket sales, jersey sales, etc.
As far as Packer fan goes, the Packers made the business decision to stick with Aaron Rodgers. Just like in any profession, if you can't return to your company, there is no reason why you can't come back with a competitor, in this case the Vikings. Packer fans should concentrate on hoping there team is better than 6-10 this year instead of getting all angry about Favre.