That depends on which game you're talking about. In Persona 5 Royal, he's free, so I'd go ahead and download him, and he comes with the game Persona 4 Golden, so there's no need to download anything there. I'd say he's not worth it in the base Persona 5 because other personas can fill the same niche. I wouldn't buy him for Persona Q because he's too high level actually to be that useful. At that point, you've already beaten the game and moved on to bonus content, and Magatsu-Izanagi is not strong enough to be helpful.