How much you like this episode depends on how much you like the detectives that make the main story of "The Killing". The Rosie Larsen plot was barely dealt with as Linden's son goes missing.
The plot reveal (possible spoilers) that is impled is that Jack is not really Linden's son, and Linden either adopted him or is guardian over him. This was not expressly stated but seems to fit into the clues. Jack is very maladjusted and Linden's fiancee has made comments over the whole season about her getting too involved "again". Linden tells Holder of her last case where a mother of a child was killed and the father did it, so the child was on his own and had nowhere to go, as he would never be adopted as he was "damaged goods". The episode also revealed Linden was an orphan too, so it seems to all fit.
The episode was a bonding episode between the two leads. This, combined with the season arc of the politician running for mayor, seems to be interesting filler to stretch the show out for thirteen episodes. By dealing with only one case, with a day for each episode, some filler is needed as there are only so many twists that are believable over the death of a teenage girl. However, how much you like an episode like this depends on how much you like the characters and care to learn more about them. To me, the detectives play their cards so close to the vest with each other that learning details about their life isn't compelling. This show is going to have to have a big ending to have made this worth watching all season.