The original "Tomb of Horrors" was Gary Gygax's first killer dungeon, published in 1978. It was, by later standards, pretty silly... but it made a big splash at the time, and is still fondly remembered by that first generation of gamers.
"Return to the Tomb of Horrors" was published 20 years later, in 1998. It's an update and a retcon of the original adventure... in fact, it /contains/ the original adventure, complete and unchanged, in all its geeky glory. But (with some really inspired retconning) "Return" puts it in a context that actually makes sense.
And it's great. Wonderful. Starts a little slow, but by the time you reach the last two chapters -- 'The City that Waits' and 'The Fortress of Completion' -- you and your players should be completely enthralled. Hell, just reading this thing can give you the willies. It's that good.
And it's huge! This thing goes on for nearly 200 pages!
Since it's a Second Edition product, you'll have to convert... and this could take a while. But IMO it would be entirely worth it. Even if you don't want to do this, just mining for the cool ideas will make this worth far more than its (ridiculously low) $4 price tag. The 'City that Waits' chapter alone is both longer and better than most $4 commercial .pdfs, and that's just one chapter out of six.
Recommended character levels are 13-16. Depending on how you convert to 3e, that might be a bit low. It's high-level stuff, and the final encounter may be epic. (It certainly /feels/ epic. By the time you meet the boss bad guy, you will know he's both boss and bad.)
It's a dark module; the subject matter is creepy, with a distinct Lovecraftian flavor in some parts. And it's dangerous -- unless you nerf it, expect heavy PC casualties. If this is not your thing, then maybe this is not for you... but, crud, for $4 you might want to buy it anyhow, just to check it out. And if you do want to run dark and deadly campaign that's really different, this is a must.
LIKED: The price. This was a $30 product when it came out in 1998 (and well worth it). Now it's just $4! Wow!
The module itself. It's OK to good in the first half, then excellent in the last two chapters. One of the best Second Edition products ever.
DISLIKED: It's a Second Edition product (and contains a First Edition module, intact), so there's some serious conversion work to be done. Also, if you want to use the visual-aid handouts (and you should), you will have to print them out and cut them up.
The First Edition module at the heart of it is... quirky. Gamers who never played First Edition may be turned off by it, so you may need to tweak it a little. Okay, a lot.
VALUE: Very Satisfied