All in all, I think this is a very good signing. The team, the city, and the player are a mutual admiration society, so it's a great fit that will hopefully make all parties involved happy for the duration of the deal.
There was a lot of talk that Plekanec could have hit $6M/season on the open market, and I don't doubt that for a moment. NHL general managers have demonstrated that they are more than willing to spend big dollars for a free agent on July 1st. When you look around the League, Mike Ribeiro and Ryan Kesler are also $5M/season players and frankly I'd rather have Pleks than either of those two. He's versatile, he works hard, and he's used in every situation. It is true that his playoff performances in recent years haven't been superlative, which is probably why he got $5M instead of $5.5M from Montreal, but clearly the Habs hope that this trend stops in the coming years.
Cap-wise, this means that the Canadiens chose Plekanec over Halak. I think that's a good move. 70 points of production on a team that didn't have a lot of production last year is a great number, and if he has a full year of a healthy Mike Cammalleri, and if they get a good 3rd man or an Andrei Kostitsyn that doesn't have his head in the clouds, those numbers will almost certainly increase. I fully believe that Plekanec can be a solid point-per-game player while keeping an overall plus +/- rating, all the while helping on the PP and PK. That type of player is pretty hard to come by in the NHL these days, so I'm glad that he'll be wearing a CH turtleneck for years to come.
Now, sign Carey!