First off, I am not an Arknights fan yet I still enjoyed it. Here are a couple of things that stood out:
The art direction is amazing. The anime is in this beautiful anamorphic aspect ratio which makes it feel cinematic, it has impactful colours and it the character designs are nice. The quality throughout is that of a movie. Almost every frame could work as a desktop wallpaper. it was something I could watch openly on a plane. However, as some have pointed out the fight scenes felt sluggish.
The story was interesting but not groundbreaking. It sets you up with the world of Arknights but feels a little unfinished towards the ends. Hopefully they'll continue where they left off in season 2 (Perish in Frost, which is a very ominous title) since they did set up a lot of big story arcs.
The music and voiceacting was A class. The OP and ED were quite good. Characters were diverse but I do wish they would have showed us more of the Rhodes Island operatives.
All in all, if you're looking for a very cinematic anime that introduces you to the lore of Arknights then give it a watch.