The Dark Below is a moody, atmospheric trpg about underdark-crawling. It has a feeling similar to Ultima Underworld, and the mechanics have just enough crunch to feel grounded, but not so much that it interrupts the close, ominous, suspenseful atmosphere.
Character creation is simple and characters fit on index cards, making this a good portable game (something to play while camping, or if you're not the driver on a road trip.) Characters also have a neat mechanic called matches, which lets them choose to auto-succeed challenges before rolling, if they're willing to spend a very precious currency.
A lot of the mechanics of Dark Below revert back to the GM making a judgement call (such as setting the TN for a roll, deciding the consequences for a failure, adjudicating when certain consequences apply,) so you may want someone who is already comfortable GMing to run this. Still, it's *not* an inaccessible game, and it wouldn't be a bad engine to learn on, either.
Setting-wise, Dark Below is extremely well-written and evocative. If the mechanics weren't also really simple and good, I'd be tempted to bill the writing as its true strength, but it really just shines all around. There's a sense of half-hidden things to the setting that feels like it belongs in a Thief game, and Dark Below also manages to hit a feeling of being otherworldly that a lot of gothic settings try and fail to accomplish.
One thing the core game could really benefit from is a dungeon-generator. Even some quick rules about making sure you keep at least one unexplored passage on the map so that the players have somewhere to go if their current direction doesn't pan out would be helpful. As is, having something like donjon on hand might be useful if you get stuck.
Alternately, you could download the expansion.
The Dark Below also comes bundled with Shades And Echoes, which fleshes out more of the setting and provides much more of a toolkit to help with modding the game to your liking. It doesn't quite give you rules for auto-generating a map, but it gives you several modes you can set your dungeon to, as well as a number of rooms, room hazards, and even npcs.
Overall, I think Dark Below is both an excellent game in its own right, and potentially one of the best pick-up-and-play options that I've seen for running dungeons from other game systems. I would strongly recommend it to anyone who likes dungeon-crawling, heavy atmosphere, tension, and just enough crunch to feel invested in the setting.
Minor Issues:
-Beginning your Journey, "do you wish to scared" to be scared, also missing caps on "your"
-I couldn't find anywhere that specifically said you're rolling a d12, but you're rolling a d12, right?