The Grinding Gear, and adventure I'll definitely not be running with my current group as I could see this being a complete deathtrap. It is an adventure for a well-provisioned, and well-balanced, group of 1st through 4th level PCs.
My first pass-through on this adventure I was more "meh"....the formatting left a little to be desired, but to be fair it was far nicer than what I've done in a similar vein. If I was to just rate this on my first gut-check I think I'd have done it a great disservice.
Had I been reading this in say...2007 I'd have gobbled it up for use with the HackMaster 4th Edition system. When I went back through a second reading this dawned on me as a great HackMaster adventure. Things that I'd frown on for my current game would have been quite awesome playing the earlier version.
This adventure is trap, puzzle, and screw with your players (only a little bit) kind of game. Many times I found myself chuckling because I could see the average adventuring group trying to overcome an obstacle by hitting it "from the side" and getting "rewarded" for it. More than once the author thought of things from this perspective and added some insight for the GM.
I don't want to post spoilers, but I have to share one of my favorite lines:
"...the answer to, "Can my character read sheet music?" is "Yes!"....."
There were a couple of points in the adventure that puzzled me (as a GM). I don't like it when there is some incongruities, or something that just doesn't seem to make sense from the game-world perspective. The author thought of this and added a "cheat sheet" that helps explain a few things. I really felt that the cheat sheet was a great touch.
The PDF is open, but that isn't as big of a deal because there is an additional PDF that contains the maps that have been tweaked for printing. If you are printing the adventure though you may want to replace the dungeon maps with the tweaked ones to save on toner.
If you're looking for a tougher dungeon crawl for your OSR game, I think you'll love The Grinding Gear. I particularly recommend it for HackMaster 4th edition games.