I can see both sides to this issue that's debated upon in every Richard Seymour thread.
On the personal level, Seymour did get a pretty crappy deal. having to uproot his family, move from the east to the west coast and on top of that play for a lousy team instead of for a contender. However he's still getting paid good money. Which leads into the other point of view; He's getting paid good money, to play a game of football once a week, for 16 or so weeks. He should know this as he made the claim of football being a business back when he was holding out - and it is indeed a business.
As for whether or not he would have been important to the team this year; absolutely. Anyone can see (and I hope has seen) what he brings to the table. Richard Seymour is a great talent who can tilt the scales of many-a-game. The way Belichick has utilized him has shown in the Patriots defensive success throughout most of the decade. But as has been pointed out in this thread already: It's all about the value. Was losing one year of Richard Seymour worth a potential top five 1st round pick? No one knows yet. It was a gamble and the results of it will not be fully known until a good 4-5 years from now. Though we will never know how much he would have contributed to the success of the 2009 Patriots season.
Personally I think he makes the difference in one or two games, but in the grand scheme doesn't bring us to the Superbowl. Many intangibles have to be taken into account other then his talent and what he offers the team. For instance how well prepared the team is from game to game and the play of his teammates. I do not think he would have been the difference-maker in the playoff game vs Baltimore either, that was a team effort that lost the game and the (potential) spirited play of one player wouldn't have gotten the Pats over that terrible hump IMO. I can sit and point out reasons why (young corps of defensive players, lack of coaching depth, etc) but I think the point stands that this team was not going the distance this year.
Just my 2 cents.