supposedly it's a sequel to persona 5 itself. i would but it for the story not the gameplay.
I would get it for the story and the gameplay. Have you seen the footage they posted on YouTube it's awsome. I would definitely get persona 5 scramble I know it's going to be awsome.
Persona 5 Phantom Strikers takes place 1 year after the main line game story.
You should get it only under two factors:
Do you like Musou games? (Dynasty Warriors)
Do you want to experience the story of the main story as it continues into a direct squeal?
It's an absolutely day 1 for me.
However, if you are unsure of the gameplay, or if you can wait a bit to learn more of the story (Or if you're a wiki abuser like me), you can always wait a few week sand see how the fan reaction is to the game in a whole.
I've done this on a few games, and dodged a bullet because of how bad it actually was.