The 26-year old is a two-down player who was a top-10 4-3 outside linebacker last season, according to Pro Football Focus. However, Misi does struggle in coverage.
The Dolphins made some splashy signings on both sides of the ball this offseason. Locking up Misi will keep a solid group of linebackers together. While the offense will get most of the attention this season, it might be the defense (specifically the front seven) that carries the Dolphins.
UPDATE: Citing a person informed of the deal, Rapoport reported that Misi's extension is for $17 million over four years, with $9 million guaranteed, possibly the largest deal for a two-down linebacker.