First off, don't even think about watching this movie-length special before you have watched the series Sword Art Online if you planned to do it. Also, if you are somehow discouraged to view this movie by its huge flashback, it's all fine, I really don't think it will become a key element between SAO S1 and SAO S2
For a duration of 1hr 40 seconds, majority of this movie is a recap. The extra footage you might want to see is like, 20 minutes of end the show. Other than that it's mostly fanservice pieces and recapitulation of the previous season of Sword Art Online.
In my opinion, it's kind of a waste of time to waste 1hr 20 mins on flashbacks. But hey, Season 2 of Sword Art Online is airing soon, so maybe it's the right time to reminiscence and nostalgizise. Might just ring your nostalgic wibes in the right moment
The last 20 minutes or so are extra footage. To avoid spoilers, I will just say: It was very inspiring. As someone who likes playing some MMORPGs and would like to make a MMORPG someday in the future (hopefully) I was all like "why are we not funding this" because none of the MMORPGs i played so far have stuff even remotely as awesome like this. Just that very aspect was what I really liked about the movie.
As the series, this movie feels like something to please younger audiences in pre/early-teenage years, judging by the plot development and depth. It's in no way hard to understand and feels simple - doesn't make one think too deep about it. Of course, it can also please audiences that would prefer action and don't like to think too deep about what happens in the anime. The plot piece in this anime is fairly simple and easy. It's really light-hearted, so if you are looking for something like that, shoot for Sword Art Online : Extra Edition! Of course, after Sword Art Online the series, that is.
