I'm a big fan of SMT games but don't really know anything about this one, how does it compare to the others? Is it related to the Raidou Kuzunoha games since it's also called Devil Summoner? I've played those + Nocturne/SMTIV/Persona3&4.
As a matter of fact, it is. The timeline for Devil Summoner goes like this:
Raidou Kuzunoha (vs. The Soulless Army and vs. King Abaddon) --> Devil Summoner (the original Saturn game that started the Devil Summoner spin-off series) --> Soul Hackers
Like SMT IV, it's a turn-based game with battles done in first-person. However, since this was originally made before Nocturne, the Press Turn System isn't in place, and like with the second Raidou game (late EDIT: just remembered that was both Raidou games for the moon phase and demon loyalty bits), moon phases and demon loyalty come into play (though unlike SMT IV, if you anger a demon during negotiations, you can't talk to that same one again, as the case also was with King Abaddon). Though leveling up demons was a factor in the Raidou Kuzunoha games, as well as SMT III and IV, demon levels are fixed in Soul Hackers. Like with games such as SMT I and II, as well as Persona 1, dungeons are explored in first-person as well (SH using random battles for combat). As the case is in SMT IV, there are different apps that you can equip to modify your options while in the field.
The combat is dated mechanically (though the battles do go by quickly), but the cast, story, and setting are quite interesting. Great music as well.