Even though he is in no way a replacement for Gonchar (they are completely different players) it's still a strong signing for a team that is clearly looking up upgrade its defense.
Michalek, 27, was one of the most coveted defensemen on the market, and has spent the past five years in the Coyotes organization where he has developed into one of the best shot-blockers in the NHL. He finished this past season 15th in the NHL with 156 blocks in 72 games, and was No. 1 in the league the year before with 272. While Michalek didn't appear to be Pittsburgh's first option on defense (that would have been Gonchar and Dan Hamhuis), the Pens probably missed a little bit of what Hal Gill and Rob Scuderi brought to the team in recent years.
He also gives the Penguins a much-needed right-handed shot on the blue line, as all but one (Kris Letang) of their defensemen last season shot from the left side.