I was on the fence on this trade, until I thought about Bennett and Avril coming in as free agents/trades in 2013. Those two put the Hawks over the top to get one Owl, and very nearly a second one. Pete is putting his stamp on the team and how he wants the team to play. Hawks have so much young talent, and mix in some veteran savvy and leadership, and it elevates everyone else on the D.
Williams was a Captain for NYG, not an honor the players take lightly. I always felt Cliff Avril had an influence beyond his play, because of his leadership qualities, and I'm feeling optimistic about the influence Williams will have. (Probably Bennett did too, I simply don't know; certainly Bennett led by example with his play) We know that Frank Clark shows up for the PLAYOFFS, if not always the regular season, so the Hawks could have a legit game-wrecker, playoff quality type defense if all this clicks and works. Clint Hurtt shows signs of growing into a legit DC as we watch.
I'm on the fair value side of the fence; really, it depends on having decent luck on the injury front. Williams has had good availability throughout his career.