The way it promotes it is by showing scenes of the unreleased episode 1, also the opening song and some other random scenes to hype the potential viewers. The VAs of Kimagure talk about Oniisan while recapping their own show. We can say it's a big promotional video for two anime series, Kimagure sure was supposed to end but the manga popularity meant new project along the way, this entry is a product of its time and should be judged depending on that.
To fill in the episode, this special has some great songs/music videos from Kimagure, you can let the nostalgia get to you and enjoy it while relaxing in your sofa and thinking about the good ol' days when Kyousuke couldn't make up his mind to choose Hikaru or Madoka, then you'll be hit with oniisan's lunatic events. Out with the old, in with the new.
I recommend it only if you're a big Kimagure fan or you wanna watch a long pv of some comedy anime no one talks about nowadays, wouldn't it have been better to simply watch episode 1 of Oniisan or watch some 2 minutes video on youtube?
What I can say about me though, this special made me watch Oniisan and I don't regret wasting so much time rewatching Kyousuke making the dumbest mistakes in the history of romcoms, Madoka is here so I can ogle her.
