I just finished SMT 1 so here are my thoughts
- there are a lot of ways to break the game, which I avoided for the first ~90% of my playthrough, then when I finally succumbed it made the game 1000x easier, just total autopilot, which was a little unsatisfying.
-I love everything about it aesthetically. even the never ending blank wall dungeons. I love the music most of all but you'll probably end up muting it after 10-20 hours because you can only hear that *thunk* sound it uses for every battle hit so many times
-you're gonna need a guide once ina while. some of them don't use the same translations as the translated ROM I used so that was fun. SMT wiki has a good item list that you'll wanna keep open because the game never tells you what any item does, the names are rarely descriptive, and even when they are, they'll usually cut off a few characters before the end of the name due to space limitations.
-mostly the guide is necessary for the parts when to advance the plot you have to go to 1 specific square on the map and uh oh, you went to the square *beside* that one, and it's a 16-bit game so your character is extremely myopic and cannot see NPCs standing one unit in front of him. missing something like that and having to double back while dealing with random encounters is grueling
-as others have said, the random encounter rate is insane. I only found one demon in the entire game with Estoma (no random encounters spell) and so I kept it the whole game. and estoma really means NO random encounters so you have to make sure you've got plenty of magnetite (the stuff you use up by walking around) because you'll have no way to restock (10k is probably enough to get through any dungeon, even the massive final one).
-in the end it was worth it (to me) but I definitely went in wanting an "oldschool" "hardcore" etc etc experience