Just finished running this with my group... well, sorta. I modified the lore of it HEAVILY to fit seamlessly into the lore of my campaign world. I'm not counting that as a negative or anything; modules get weird after first tier. I loved that there were details like that the walls were covered in murals but without the specifics - good writing prompts haha. I spent sooooo much time creating an entire history of three fey realms and everything to fit into and flesh out scraps that I had previously cobbled together in my world. Had to add a bit of time warp / dream world stuff into the mix too for my own reasons, but it also helped explain how the drow (re-skinned as winter eladrin and kenkus in my runthrough ;) ) were inside already if the vault only opens for one day. How/why there were so many undead/fiends (or agents of lolth for that matter) wasn't fully explained, but I just came up with my own reasons for that since I conveniently have a powerful archfey messing around with undeath/immortality in my campaign anyways.

This module was super easy to read though, and my players thankfully went to most of the rooms in order so there wasn't too much jumping around in my notes. Took us about 5 sessions (~3h each?) to get through with our roleplay heavy group, though since every wall except the crypts was covered in murals of my creation, there was a lot of extra padding on my end. Some people and places that seemed familiar to certain characters... so time spent discussing that sorta stuff in character as well. Not to mention they decided to team up with the drow-reskinned-as-eladrin-and-kenku... and also brought the xorn along... so the end battle with the firefly guardians until the end of the module took a LOT longer than it would with a more hack'n'slash group even though the firefly guardians themselves went down pretty quick, since there were so many moving parts and things to wrap up, like the sudden and inevitable betrayal of the eladrin group lol. All in all I really enjoyed dissecting this module. For 4 (normally a bit OP) players at 7th level, the APL 7 suggestions seemed very appropriate with average HP for the monsters all the way through UNTIL the final battle... though that was likely due to me not buffing the firefly guardians up enough to counterbalance the entire troupe they had brought along by that point, and could also have been that they had gotten fairly lucky on the max HP/STR reduction things up to that point. The firelfly guardians seemed to go down pretty fast, would have liked to see some legendary actions or something in the stat block, or more HP (should have stuck with my gut and maxed the HP out on those). Made me glad I had "boss battle part II" in store when Faradanna and my reskinned archfey faction tried to take the goblet that one other character needed to get for their own character development/backstory/RP reasons. All in all, I really enjoyed running this module and the opportunities I had to tweak things as well as the opportunities the players had to interact with things. This was my first time running a true "dungeon crawl" type, and it was exactly what my campaign needed to shift gears for a bit.

A Solid One-Shot Dungeon Crawl: This is a no-frills short dungeon for Tier 2 characters, and judged by that standard, it won't disappoint. Its adventure hooks have enough flavour to be interesting, while being generic enough to allow the adventure to be dropped into almost any ongoing campaign. (I used it between chapters 3 and 4 of Ghosts of Dragonspear Castle.)

It presents a short dungeon, themed around the intersection of fey, undead, and insects. Encounters feature monsters that don't feel overused, and have good combat balance. The writing is clear and easy to understand. Relevant information is presented in intuitive places. Everything is workmanlike, which I mean in a positive way.

It's not perfect. The dungeon map may be convoluted, but the adventure is actually very linear, with few chances to hit encounters outside of the necessary order, and little in the way of optional areas. The backstory is confusing, but little of it is actually relevant to the adventure. Some of the module's best ideas are semi-wasted by being dependent on the use of specific adventure hooks or alternate conclusions. And though the boss monsters look like unique new creations at first glance, they turn out to be awkward reskins of existing Monster Manual staples whose abilities don't really fit their new appearances.

Still: it does what it does well, and it doesn't overstay its welcome. Those who want to push the boundaries of the 5E one-shot module should keep looking. But those who just want to keep their Tier 2 party entertained for a session between meatier outings will find everything they want here.

Additional advice: The module features quite a lot of incorporeal foes who resist non-magical standard weapons. Make sure every character has a reasonable option for attacking these, to avoid a frustrating experience for some players. Also, despite much talk about a special puzzle designed for a bard, the puzzle is unfortunately of the poorly designed sort that doesn't actually offer any interesting gameplay around it, and there's no substantive content for bards beyond merely checking if the class is present. Finally, a piece of design oversight means that almost every monster in the dungeon will be relying on their darkvision to see the players - except that the dungeon specifically notes that darkvision is magically neutralised within the area. Best advice is to ignore this business about darkvision not working and run the dungeon in accordance with whatever your normal policy on lighting and vision is. (A number of monsters that rely on shadows to be effective will also be rendered trivial if you encourage PCs to cast Light on every member of the group in this way.)

I ran this as a one shot for four experienced level 6 characters and we had a fantastic time! There was enough deadly adventure to keep them on their toes and nervous about entering each area after the first encounter with the shadows.

I tweaked the encounter option with the archfey to have him trail the party as an invisible pixie from a distance allowing him entry to the final chamber where he attempted to abscond with the goblet - a high action escape ensued and everyone was hyped afterwards. I am running my second group through this content and in my experience it takes a fair bit longer than the 4-6 hours noted. It was about 6-8 hours for our session pace in the first group

My group and I really enjoyed this adventure as part of our larger fey-based campaign. The adventure is well-organized and pleasing to the eye, and presents a nice variety of hooks and ending possibilities. My group has 3 players (paladin, warlock and ranger) at level 7, so I mostly used the suggestions for APL 6 to make up for the lack of a 4th player. They very nearly had a TPK in their very first battle (against the group of wraiths and shadows) due to some absolutely abominable rolls, but they clawed their way through it and the rest of the tomb was at just the right difficulty level to be challenging but not party-destroying. It took way more sessions than I anticipated to get through it due to extended episodes of the group's RP antics, but in our group we always consider that a good thing!

I DM'ed this game with a group of teenagers who had never plyed the game and they really enjoed it. It was a great experience for players with limited experience.

Goblet of Fireflies is an extremely well-written adventure with unique and interesting lore and mechanics that truly set it apart from your standard dungeon crawl fare. Geared to Tier II adventurers, the module is presented with easy to follow instructions for scaling the encounters to match a party's level. The magic items contained herein are fun and the maps are great!

There are a few areas that could use a bit of attention in terms of clarity (particularly in the Ballad of Light section) but a DM with basic knowledge of the rules could certainly patch any gaps in understanding on their own.

Overall, I highly recommend The Goblet of Fireflies and cannot wait to run it myself!