Recently I read a blogpost about The Tomb of Horrors.
The blogger had ever thought it was the worst adventure ever published and used all of his wit to demonstrate it.
He said how it caused an almost instantly TPK, how this created arguments with his players, how he had to work behind his DM's Screen (fudging dice or changing something) to make the adventure easier after he had given a second chance to his players, and how, last year at a convention, he, as a player, had managed to be the lonely survivor (being the only one who already knew the adventure) after a few minutes, and then took the loot of his fallen comrades and went out.
I don't want to argue with that guy (TPK and arguments were troubles because his very young age, 12, and in adulthood he's still a child due his behaviour at the convention) and so I only want to share with you my personal considerations about Tomb of Horrors.
I played twice and I know it's an unique adventure. Gary Gygax put at extreme consequences and conditions the art of dungeon crawling. Everything is an excess: the save or die traps, the traps where you die without even a saving throw.
Gygax devised the Tomb as an amusement, an impossible adventure for the players, who, in fact, could end it only if they had previously read the book.
You must play it as well. Consider it a great amusement: a lot of laughs for some nights and you'll reach its goal.
Don't run it as part of your regular campaign: it's too different from any other adventure and can only cause you headaches.
If you're planning to run it in your campaign, consider Return to the Tomb of Horrors instead.
It was written in the late 90s as a reboot and it's a much more detailed adventure.
PCs will face giants, a lot of undead, a cabal of necromancers, an have a long trip in the Plane of Negative Energy, other than enter one more time in the Tomb itself.
The School of Necromancy can be easily put in every campaign world, and it's very well described with a lot of interesting NPCs too.
I ran that adventure back in 1998 and I can confirm I had a lot of fun and spent many funny hours.
Try it and you'll be satisfied.