I feel like I should probably leave a review, considering how much time I have spent playing srb2p.
I didn't get the opportunity to play more than 2 hour of multiplayer overall, and I barely touched the modding support, but despite that, the amount of content available in vanilla srb2p is enough for quite a big amount of playtime. Honestly! If srb2p got cancelled tomorrow, the amount of content found here is, in my opinion, enough to consider it a full game (despite it only being a demo).
The battle system is a mix of persona 3's 3 physical types, persona 5's main battle system (minus being able to baton pass to teammates that have already had their turns) and elements, persona Q's sub persona system and boost meter (and link attacks) and the SMT buff system. The added Rogue mode means that you can't just remember the affinities of enemies, so you do have to figure out how to beat an enemy the first time you encounter it. Shuffle time is back, too. It's a mashup of everything great about the various persona battle mechanics.
srb2p also include voice acting and quite a few easter eggs. I would add more, but I don't know how much more time I can say why it's good without it sounding like a bigger version of the description tab
tl;dr : srb2p good game, very fun. Also made me try srb2