Writing-wise I’d say the world building is arguably better than the “plot” so to speak. The world is well-built and beautiful - areas are sectioned off into zones with loading screens, so it’s not as open world as one might want, but I don’t have an issue with it. Playing with friends (or making new friends through the game!) certainly makes it better, though. You can play it as a single player game for the most part if you want to - you are required to group up for dungeons and boss fights, but the matchmaking for that stuff makes finding a group effortless. After you own the game on multiple platforms, you also only need to pay one flat subscription fee (as in, PS4 and PC don’t require separate subscriptions). Everyone plays on the same servers, so as long as you use the same Square-Enix account (which you are required to set up when starting the game) on all platforms, your character goes with you across platforms.
The base game and expansions have to be purchased separately for each platform (PC, PS4, Mac).