If you already watched the drama, you will probably be disappointed in the special. I didn't like the drama's ending so I thought that the special will fix it and probably make it up for that unsatisfying ending but no the special wasn't any better, it even made me angry and regretful for watching it : another open ending with a non special story made me want to finish it quickly and get over with it.
This isn't what you're looking for and don't dream to have a better ending than the one from the drama!
I really wish I hadn't watched this special. I got all emotionally worked up about the ending of the series only to be dragged through all the same junk over again! It just made me really sad. I also felt like there was a tiny bit of inconsistency in a few of the characters and that bugged me. I would recommend just sticking to the ending of the series and forgetting about the special. Not worth it.
I was kinda disappointed with this special. It should be better called as episode 12 than to be a special episode because I don't think something is special in the story of this one. It was more like a summary of the whole 11 episodes, a bit of a new story, with an open ending. Really disappointing.