I downloaded (and had initially intended to buy in print) this adventure based on a smattering of online acclaim that it was the adventure to use to teach players new to the OSR style of gaming. It is system neutral and can easily be ported to the old edition or retro clone of your choice. This portability is the only good thing I can say about Tomb of the Serpent Kings. (Besides it being free to download.)
For all the set dressing involving sorcerer serpents and such, this is a pretty boring dungeon. It's got all the complexity of the sample adventures you can find in the old red Basic player's manual. The author intends to teach "lessons" about OSR-style play, but mostly these lessons are taught by smacking the players in the face with it and shrugging its shoulders, because OSR.
While there are some nonstandard original monsters at the end of the dungeon, I found it to be too little, too late. In fact, the group I tried running it for (Pathfinder players interesting in trying old school rules/playstyle) were ready to quit not even halfway through.
There are better adventures to use if you're planning to teach some new players the ropes. This seems couched firmly in the idea that OSR play is hardcore and the players have to learn the hard way. The tone comes off as condescending and spiteful. ('Dungeons contain undead, and undead are dangerous!') Hard pass on running this for players new to OSR style stuff in the future.