One of my favorite adventures, in part because of the extensive wilderness adventuring (especially in combination with WG4 as noted already): there really were very few true wilderness adventures published for 1e, and this one helped to set the stage for many of my own creation later.
Like Sep, the demonic lore made a big impression on me as well, although there are some subtle differences between the rules for protection circles and their accompanying spells as published in S4, Unearthed Arcana (1e version of course), and their first appearance in Dragons 56, 67, and 68. Well-worth checking out!
I ran the adventure multiple times, especially after WG4 was published. None of the groups I DM'd were ever able to claim Iggwilv's prize, nor to free her dreaded daughter from stasis---the dungeon was just too brutal. Twice I had groups go over the falls on level 1, never to be seen again.... The four prison demiplanes were also a lot of fun, in particular the pentacles one. The new monsters were a wonderful challenge, and I made immediate use of derro and several of the others not present in the module itself.
Lastly, S4 is probably the first Greyhawk module to really inspire me to start digging around for more history on the setting itself: its ties to Castle Greyhawk (via Fraz), Iggwilv's legend, the cambion (hinting at Iuz's ancestry), etc.
I'd love to expand on S4 further (as my page about the module details:
http://www.greyhawkonline.com/grodog/gh_s4.html), in a manner that RJK did with Maure Castle in Dungeon 112! (Gygax of course would be the better one to do this, but if he's too busy, I'd happily oblige!

)