What is it people like about the SMT series? How does it differ from the Persona series? How do the spin offs differ? Like, what's the difference between Summoner Survivor and Digital Devil Saga and such?
Missed this, but the things that connect all Megami Tensei games are the demons and (most of the time) demon fusion mechanic, also most post-Nocturne games have a variation of its elemental weakness-exploiting battle system called Press Turn.
Different people like different things about these obviously, but their dark tone, atmosphere and themes are usually what draws people in, also playing these guarantee that you'll at the very least get a great soundtrack and fun combat. As for some comparisons, Persona is more character-driven, have a calendar system with social sim elements and combat is relatively simplified while mainline SMT is all about their worlds, the conflict within them, a darker tone and more challenging difficulty with a deeper combat system. Devil Survivor are strategy RPGs with great focus on decisions and Digital Devil Saga are more like traditional JRPGs with a set party and no demon recruiting or fusions, fairly story and character-driven too.
I bought Soul Hackers back in the day because the atmosphere looked cool and I wanted to try a SMT game. I couldn't do it :( These games are way too slow and I didn't find the gameplay too engaging.
Soul Hackers has to be one of the worst introductory titles out of the localized games, it's an enhanced port of a very dated Saturn game that's too clunky in many ways. The game's main draws are its atmosphere, setting and soundtrack, but you have to tolerate a lot of hardcore dungeon crawling with a weird (and noisy!) interface and some pretty dull combat to enjoy any of it.
If you ever give it another go, try SMT Nocturne on PS2 or Shin Megami Tensei IV on 3DS, those are more representative of what's so good about the series and the former is one of the greatest JRPGs out there.