One of the best dungeon/bloober RPG's I ever played. And I played stuff like Wizardry 6, Eye of Beholder 1-2, Lands of Lore, Legends of Grimrock 1&2, Might and Magic 7 & 10, Labyrinth of Refrain & Galeria, and many others.

The story is very Anime japanese high-school teen, so if you don't appreciate the genre you will not like it. Your characters are students in a high-school and half the story scenes are displayed inside the school, but the action is 99% done outside the school as some sort of special super secret evil fighting team.
The story is super fun, with lots of comedy and serious moments, as expected of an Anime. For example you have the African-american English teacher in the school who is always providing comic relief due to cultural reasons, leaving you laughing on the floor.
Amazingly the story makes sense and is very consistent (something very rare in most games).

The gameplay was easy for me (I party wiped less than 10 times), but might be harder for first comers to this genre.
Some of the puzzles/missions are old school hard, mainly due to having to search the map for the location of the event to trigger, but there is a spoiler free guide in steam that you should use to remove that kind of frustration.
There are also gate spells to return to base or teleport around in the dungeon, removing a lot of tedium in this kind of game.

Combat was fast, with easy random encounters never taking long. While the harder random encounters still being dangerous and requiring my attention to avoid losses. Combat can be deadly, and it's hard to resurrect in the dungeon for 95% of the game, but the game provides you the systems to avoid death with many defensive options. In my game half my team played defense while the other half dealt damage. Operation Abyss was also one of the few games that rewarded me by investing in HP. Enemies can hit quite hard if you make a mistake, so I pumped the HP stat to 15 in every char at the beginning like guides recommended but at the end of the game once I maxed my offensive stat, I was dumping again into the HP stat to make my life easier.

There are two modes at the start of the game. One uses static art images for your characters and another (more ugly) allows you to see the EQ your wear reflected on your characters. I used the static images one and it was fun, but the other mode where you can see your char EQ is a lot more interesting as you can play dress up doll on your chars.