Playing this game for a year now, and so far can't really add much to what already been said, gameplay is solid, new mechanics help keeping gameplay fresh.
With 8 available classes and multitude of subclasses supports wide range of teams that allows virtually limitless possibilities.
As well as new Rogue lite mode that doesn't require any in-game energy can let you entertained for a while while also giving you valuable rewards.
In my year I spent at most 25$ into the game in buying monthly pass and some premium currency for skins.
Gacha aspect allowed to get my fare share of powerful 6*.
Although to developer team credit in balancing the game you absolutely can beat the main game without any meta characters. And it is generally advised by the community to not rush for rarity because all characters have their use. Not to mention you can always take "support units" which is another player characters that you might not own to help or to just test.
Story and lore are very deep and has it fair share of dark and sometimes downright depressing moments can't go really into details without spoilers (just go read Children of Ursus)
And music. . . To make things short there's a reason people saying that Arknights exists to be a promotion for their own music company and its absolutely justified there's something for everyone.
TLDR go play it its good.