Not great, but few RPGs for the Sega were.
First off, even though it has the word Shining in its name, this game does not seem at all related to the Shining Force games (except for the presence of Dark Sol and the "special deals" option in the stores, but those seem like tenuous connections at best). After coming to grips with the fact that this game will not be a masterpiece like the Shining Force games, it's much easier to enjoy as a fairly straightforward RPG.
The gameplay consists of turn-based combats, and the only area to really adventure in is the gigantic labyrinth that is near the town. Although the labyrinth is huge, and serves it's purpose as the only dungeon very well, the fact that you are always adventuring in the same place, with no changes except for which floor you're on does become irritating after a while. One interesting aspect of the game is that it takes place from a 1st-person perspective as you move through the dungeon, unlike the usual 3rd-person Birdseye view that is used in almost all early RPGs. And that about sums up Shining in the Darkness...it can be a good RPG, and definitely is worthy of curbing RPG withdrawal, but wears thin after a while and lacks much staying power.