One of the first times I ever played D&D, actually. My brother's friend Greg was running the mod, I got to sit in on one of the sessions and played the NPCs of the cleric and the thief (Quillan and whatever the cleric's name was). I remember being the first one to "skulk through the barracks" as we raided the installation in search of information. I think we may have even gotten to the confrontation with Xanathon (first time) before we ended the session. I wasn't able to finish the game with them, though.
I ran it several times as a DM myself, years later. One of my favorite modules- as someone else noted, it's very open to player choice (not a "railroader"). The plot can be as complex or straightforward as the DM and Players choose. Very different from most adventures at the time- though there is a dungeon in it, it was not really hack 'n slash, nor a dungeon crawl. It was an investigative mystery, with potentially very heavy roleplaying. There were some technical glitches, of course (if the players go right to the ducal palace instead of other areas first, you could potentially have some troubles), but nothing that isn't easily solved.
One of the other things I liked about the module is its transportability. Though set in a particular town and region of Mystara's Known World, it can easily be shifted to other regions or worlds with a minimal bit of reworking- which is exactly what I did in a later running of the module, when I moved it to the Darokin/Atruaghin region of Mystara.
