This film, unlike 1st and 2nd, have a brand new original story using elements from other media, such as the manga Nanoha Force (for Bardiche and Raising Heart new design) and the PsP game Gears of Destiny, as the Florian sisters are introduced here. It's the first part of a two-part story.
The movie itself is mediocre or at least not good on its own. Too much action but there isn't enough time to introduce or develop characters, hence not a good entry point for someone who wasn't familiar with the franchise. The pacing is rushed as if a season was condensed into 110 minutes, and it ends with a cliffhanger for the sequel to solve. Generic Magical Girl x Mecha in its purest form (NOT Moe), no subversion, no parody, nothing degenerate either, just uninspiring. Looks like something from the 2000s, or an addition to a franchise that has passed its golden days.
I'll keep this brief and spoiler free since most probably have not seen the movie yet.
Unlike strikers which sidelined the main cast in season 1 and 2, and vivid which basically almost changed the main cast entirely, Reflection retains the main cast from season 1 and 2 therefore it was an instant plus to me. The main cast has great chemistry and therefore is always a joy to watch them interact.
Reflection's tone is similar to that of season 1, 2, and 3 (Strikers), not very lighthearted like that of vivid. Although not as well executed as As, the villains (well most of them)
have believable motives which you can empathize with. The movie does end on a cliff hanger leaving many loose ends that will hopefully be tied up in the Nanoha: Detonation.
Overall, if you are a fan of season 1 and 2 or movie 1st and 2nd of the nanoha anime, you will most likely like Reflection.