I think people tend to stick to a certain perception of a player, so I I can understand why some people see this as bad deal. When Petry got here he was a mobile but soft defenseman. His first full season in Montreal he was being completely outshined by Markov, Subban (a guy roughly his age) and was at around the level of a much younger Nathan Beaulieu. He was 27 years old back then so I guess this perception of a soft, shaky and not very productive 2nd pairing-at-best defenseman just stuck, because it was logical to think we were seeing a finished product. But Petry consistently improved every season since. With the arrival of Claude Julien, his decision-making considerably improved, so that smooth skating was now put to use a lot better, both offensively and defensively. The last two years (coinciding with Luke Richardson's hire as defensive coach) have seen even more marked improvement. He's a lot more assertive now.
Petry is a late bloomer and doesn't play the most spectacular game, so he'll never be "the hotness" for fans. But this is a very good deal for the team.
Great skater, good size, smart player, great offensively and defensively despite often being paired with crappy, AHL defensemen. A quality veteran who is well liked and respected by teammates. Durable player who has missed few games and logs 23 minutes a night.
6 million dollars for that seems a bargain to me.