first a disclaimer - I've watched it right after playing the game, which influences my opinion.
Long story short, it's specifically for those who already played the game, but want a slightly different true end. I take it as yet another route, that was not there.
On one hand I like how they ended it in the anime, but I don't think that's how this story should end - it's the same as rinne's "bad" ending in summer chapter, a nice ending, but not the true one.
On the other hand, it's very rushed - to the point that not knowing the game it would be hard to understand what's going on. On top of that some of the changes to the story made things worse - good example is the scene in ep1 where Setsuna meets Karen on the boat. In the game it made logical sense, in the anime it's like "why did he even go there?".
Long story short, if you liked the game might be worth watching. If you haven't played - play the game first.