The short version: at a price of anywhere from $0 if you're strapped to $3.00 if you go about what's average, this module is well worth your time and money.
I like monsters that don't get much love, so one that featuers ankhegs is already looking great in my book. On top of that, though, this product delivers exactly what it says - a punchy, fast-paced 5e oneshot that you can do in a single session (assuming your sessions are of a solid length, y'know, a few hours or more, I think is what you'd need) - excellently. First, the presentation is pleasant and adds to the experience of reading; presentation is a critical part of design, and this module nailed it in a way that a lot of other 3rd party stuff I read doesn't manage to.
What I love is that eading the module gets you exactly what you need when you need it. You can actually start at the beginning and get a great intro and synopsis that tells you, the GM, what's going on without having to deal with any vagueness as too many adventures - even official adventures! - I've seen done. There's some simple hooks with advice on how to run them, too (though I do have a quibble with the 3rd hook's recommended encounter of goblins or an owlbear being a bit ill-suited to foreshadowing the adventure ahead. Maybe goblins or an owlbear with evidence of acid damage, fleeing the cave, thus giving signs of the ankheg infestation? I dunno).
The NPCs are straightforward and useful; including the daughter Meeka, I think, helps give some atmosphere and depth to the farmer, Markus, and I find that PCs and players are often motivated by children, so I suspect Markus having a daughter will motivate the players to rescue him, the farm, and the community.
The caverns are well-put-together by my standards; there's a map (not being a cartographer, I really appreciate getting maps from modules), and, perhaps to some degree more importantly, there's also a description of the nature of the tunnels. You're told how uneven the ground is, the height of the ceiling, and so forth. Very useful stuff for both atmosphere and running exploration and combat. I also like how it's an ankheg den built into a former kobold mine; I just like dungeons that are "repurposed." Things like the how in Against the Cult of the Reptile God, the cult's lair is the touched-up mud tunnels of troglodytes or how Lost Mine of Phandelver's Cragmaw Castle was once the seat of a past era's ruler before it was taken over by goblinoids. I feel like it gives the place a little more history and makes the world feel more alive. Of course there's isn't just a dungeon here: it's a place that served a purpose to people in the past.
Encounters are very well-detailed; GMs are given information and advice on how to react to different actions by the players as well as how to run the monsters - their tactics and strategies, and such - in ways that I think have great potential to make the fight dynamic and give them their own personality, in comparison to other fights.
As for flaws, I mentioned my quibble with the suggested encounters for hook #3 above already, and that's my main complaint with design. Also, I don't think the ankheg eggs should require attack rolls, especially with AC 0; I'd have just said that the players can smash one as an Attack. Aside from that, there are a few presentational matters that require some touch-up, though it's pretty nit-picky stuff, and honestly not mistakes I haven't seen in a few official books.
- On page 6, "Due to the ankheg's tremorsense" should be "Due to the aknhegs' tremorsense" because there are multiple ankhegs.
- On page 6, "have them make a Perception Check" should be "have them make a Wisdom (Perception) Check" in order to be consistent with 5e.
- On page 6 in the Skill Challenge, the module states that on a failed Perception check players can collect up to 50 gems, though it seems like it means to say 50 gold pieces worth of gems.
- On page 7, "Draconic" should be capitalized but isn't.
- On page 7, "The remains of the cow that was drug down" should be "The remains of the cow that was dragged down," as no definition of "drug" is not the past tense of "drag" (at least, insofar as I and Google seem to be aware).
- On page 9, "they can do so with a Heal Check" should be "they can do so with a Wisdom (Healing) Check" in order to be consistent with 5e..
(For the quibbles about the style of writing what skill the check is, there is a possibility I have simply misunderstood the module's intended style of presentation; if so, instead what needs to be changed is page 8, which uses "Intelligence (Investigation) Check" rather than just "Investigation Check".)
Like I said, though, these were nitpicks. Overall, I can heartily recommend the adventure, especially if you pay average price. If you need some bestial monsters for your party to fight and feel good about beating, this is going to be a great product.