SAO is a series you either love or hate, and remains one of the more divisive anime out there. I personally love it - while recognizing the reasons why others do not. That being said, even those critical of SAO will want to give Ordinal Scale a watch. It's a breath of fresh air to SAO, with greatly improved action content and development/screentime more evenly distributed among the series' characters.
The switch from VR to AR is interesting, especially since we get to see the characters struggle to adapt to "real life" combat and the blending of the virtual and real worlds. The plot is also full of throwbacks and added lore to the Aincrad arc, something that most fans will agree is welcome. I will stop short of calling it great however, as the movie does have some hokey moments that require a little suspension of disbelief to enjoy, and there was a greater focus on action scenes over plot than there'd been in the original series.
The art, while phenomenal, felt quite a bit different than the original - not that surprising since it's now 5 years old. None the less it was really well done, especially the fight scenes which felt much more dynamic than those in the original series did.
I was really excited to see LiSA (who gave us both OP1 "Crossing Field" from the first arc and ED3 "Shirushi" from SAO II) returning to the series, and she didn't disappoint. Her OP/ED "Catch the Moment" was fantastic, as were the rest of Yuna's songs by various artists. Heck, we even get a song from Silica in what is undoubtedly her cutest moment in the series so far. Some of the best BG music from the original series is re-used, especially whenever Aincrad lore is invoked, and it's great.
SAO is primarily a character-driven series, and Ordinal Scale does not disappoint here. A significant amount of screentime and development is given to Kirito and Asuna's relationship, but also to the original cast who many feel were unfairly sidelined after the first arc. Klein, Egil, Liz and Silica all get their moment in the spotlight, and minor characters from every arc are brought back for brief roles throughout the movie. The OCs of the movie, Eiji and Luna, are really well done and give us a whole new insight into the tragedy behind the frontlines of SAO.
Sinon also gets some screentime with Kirito, something those who prefer that fanpairing will be happy to see. Leafa is mostly written out of the story, which is the only thing that keeps this from being a 10/10 in my opinion. However the writers have a good justification: with OS being an AR game based on real life sword skill, kendo champ Suguha would've stolen the show!
Enjoyment and Overall Rating: 8/10
The short of it is: if you loved SAO, or at the very least loved the Aincrad arc of SAO, you will love Ordinal Scale. Even critics of the series will enjoy the action scenes of the movie compared to the original and the fact that the movie is not purely focused on Kirito. It's got the perfect balance of action, tragedy and romance that made the original SAO such a hit, and is visually and musically stunning. I definitely recommend it, but for those who absolutely cannot get over the cornier parts of the plot of the series, you may enjoy it less than I did.