This will be coming from someone who is overwhelmingly hyped for Persona 5, and a large fan of Shin Megami Tensei in general.
As a standalone OVA the story is a bit less than you'd expect. It has its interesting points and a decent, if a predictable, twist. It certainly sets up a good atmosphere that never fully breaks, but that's about it. There's no real pay off in the end as it sets us back to the Velvet Room at the end, making it a "possible" path rather than one that we should just assume happens in the future game.
The characters are alright, if you've been keeping up with the trailers. As a half hour OVA should, it wastes no time on introductions and goes right into the plot. There's certainly potential for the characters in the game, but they're rather one note in this OVA, to the point I'd argue the trailers had more character to them.
The sound and music, as expected of an Atlus product, is fantastic however. Most of the music is from the game (I'm assuming, as it's in the trailers), and it fits well with the fast paced action they try to form. It's also just very nice to listen to in general.
The animation however is extremely poor at times. Never to the point that I couldn't look at it, but definitely to the point that the impact of scenes was lessened. The best example of this is the Persona summoning scene. While the scene itself is very cool, showing the Phantom Thieves working as a team and showing off their powers, the animation is just too poor to leave the impact it was supposed to.
Designs over all, also expected of an Atlus production, are amazing though. Characters look fantastic, powered or not, and the Personas look much better than Persona 3s and a bit better than Persona 2 and 4s in my opinion, especially Arsene and Goemon and Kidd. The world they enter into seems much more interesting than the Midnight Channel, and possibly more than Tartarus, making me think more of that in SMTIII and IV, which is a very good thing. I'm a bit disappointed that the enemies fought end up being generic demons instead of something unique. At the very least, while the grunts were immediately recognizable, I can't immediately place what the boss Demon is even though I know I've seen it a few times, so it's less used at least.
Overall, it was a decent OVA that did its job, it got me more excited for Persona 5. It gets a 7.
Also, shoutout to the Devil Survivor/Persona 1 reference in the Kazuya/Naoya brothers.