In Darkness over Daggerford, you play a simple mercenary. You start out as a bodyguard for a tailor, but then your caravan is attacked near Daggerford (which is located between the famous cities of Waterdeep and Baldur's Gate), and you get sucked into a larger conspiracy. From talking to the townspeople, you learn that lots of bad things have been happening in the area lately, including the death of the governing duke, and you eventually decide to explore the surroundings to see if you can figure out what's going on. This leads you to battle a good chunk of the official Dungeons & Dragons bestiary, including vampires, werewolves, orcs, trolls, dragons, and more.
Darkness over Daggerford has a large collection of quests - I counted about 50 of them when I played through the campaign. In some ways this can be perceived as favorable, because it gives you lots of things to see and do, but in other ways it's detrimental to the experience. The campaign lasts about 25 hours, which means you're completing a quest every half hour. As a result, you go somewhere and kill a few quest-related adversaries and then you return, or you go somewhere and you talk to a quest-related NPC and then you return. The quests are all well-written (and sometimes funny), and there are almost always different ways to solve them, but they're all short and sort of inconsequential, and they rarely feel like they're connected to a greater whole. Instead of Darkness over Daggerford mimicking a novel to read through, it's more like a collection of unrelated short stories.
Also problematic is how the companions work. Unless I totally missed something, there are only two companions available in the game, and you can add them both to your party at the same time. Sadly, while you might think that the low number of companions would mean that they're integrated well into the story, that's not the case. The companions don't have an agenda or any needs of their own. They're just happy to follow along with you, and they don't care how or if you solve quests. Infrequently they add dialogue while you're adventuring, but mostly they only talk to you when you gain a level, which doesn't happen very often (I only gained six levels when I played the campaign, and the companions stopped updating their dialogue after the fourth). This is easily my least favorite kind of companion. Worse, you might actually be better off playing without the companions, simply because the combat seems to be tuned to you playing by yourself, and with a full party the fights are embarrassingly easy.
However, other than the companions and the scope of the quests, just about everything else about Darkness over Daggerford is a positive. Ossian added an overland map feature to make it easier to move between areas, they included some new tile sets, placeables and music to enhance the look and feel of the module, and they interjected a lot of humor into the proceedings to keep the tone lighthearted and fun. There is also a whole bunch of replayability to the module. Despite being a fairly thorough player, there are some places I never figured out how to get to (like a tower surrounded by an impassable chasm), and a couple of optional quests I wasn't able to finish, and I think these things have more to do with the type of character I was playing rather missing important clues or NPCs. For example, I noticed lots of skill and attribute checks in the dialogue (especially for the search skill) and perhaps with a different kind of character I would have received different options and responses, and unlocked different things.
Darkness over Daggerford is the first and probably only free module for Neverwinter Nights that I'm ever going to play. For all I know it's the best free module ever released, but since I don't have a frame of reference for this, I'm going to compare it instead to the premium modules that I've played. Even there Darkness over Daggerford holds up pretty well. It's longer and better written than most of the other modules, and while it doesn't serve up many (tricks) (there's one mirror puzzle but that's it), it shows a solid use of the Aurora toolkit, and after two patches there don't seem to be any bugs. Plus, you can't beat the price. So if you're one of those people still playing Neverwinter Nights, then there isn't any reason not to check out this module and see how it goes.