First, I'm still surprised the Patriots made this move. Yes, he isn't being a good teammate, and he wanted a new contract. But he's also a deep threat who can spread the field and open up the short passing game for guys like WR Wes Welker. But as Bill Simmons pointed out, the Patriots defense is young and probably a couple years away from challenging for another Super Bowl. By the time that happens, Moss isn't going to be around anyway.
Second, while Moss really fills a need for the Vikings, he isn't the best fit. They need a replacement for WR Sidney Rice, who's job it was to run the slant route. That's not what Moss does best, as he's usually running deep. He's duplicating the role of WR Percy Harvin (unless Harvin is going to start running short routes in the middle of the field). Adding a receiver like WR Donald Driver would have been a lot more valuable.
Eric at Daily Norseman has an excellent statistical analysis that points out the slight statistical bump by his addition (though it does add about 10% to the likelihood they make the playoffs). Still, as Aaron Schatz at Football Outsiders points out, Moss clearly has something left in tank and should help them this season. If all it cost them was the promise to discuss a contract extension, and a 3rd round draft choice, that's not a high price to pay for a team trying to win now at any cost.
Third, how much should Moss worry the Green Bay Packers? We'll it's not going to be like 1998 all over again. The Packers cornerbacks are taller then many teams, and while not as tall as Moss, it won't be a significant mismatch. Right now, until we can see some games with Moss in a Viking uniform, we'll have to wait and see.