I am a member of the "If-it-ain't broke don't fix it" crowd as well as the "If it works, run it into the ground"
While there have been other modules that I've enjoyed more, I've run the Clearing of Castle Caldwell portion of B9: Castle Caldwell and Beyond over a dozen times, teaching dozens of people, young and old, how to roleplay.
Fun Fact: I have never owned an actual copy this module. I have owned, however, numerous copies of the compilation B1-9 In Search of Adventure.
Of the adventure paths presented in B1-9, clearing Castle Caldwell is the most obvious choice for a group of brand-new adventurers (and players). It's local for the PCs, so they can head home and rest if it gets too tough, the pay is very good, even for gold crazy Basic D&D, and it's only a one-story structure (mwhahaha). The encounters present a chance for combat, role-playing, negotiation, a chance to run away, and even an early example as to why basic encumbrance rules are needed in D&D.
The main castle is a great opportunity for a group to figure out their tactics without getting turned into a bloody pulp. The second part, the Dungeon of Terror underneath is a classic bit of D&D railroading, but in the end, I've only ever lost two characters down there.
I treat Castle Caldwell as a sparring match in a boxing ring to prepare someone for the real thing. The next adventure I can take off the headgear, but they're still fighting some journeyman with a glass jaw and a 3-7 record, but if they're not careful, that left hook will leave them permanently on the floor.
Besides the Castle and the Dungeon, B1-9 also includes The Great Escape from B9, which requires the right group/mindset to play. I've also used The Sanctuary of Elwyn the Ardent in my "College Game" as a heart-ripping railroad (as recommended in B1-9) to transport them to B3 Palace of the Silver Princess.
While it's almost universally panned in every review I've ever read (and some questionable editing in the B1-9 compilation), what people fail to notice is that the majority of TSR product (and beyond) are "gimmick dungeons." Some, like B2 are lauded by all, and yet, when you pull away the gimmick, monster location and ecology and just as bad as they are in B9. Castle Caldwell is a stock dungeon, one that only got respect or attention in a random issue of Dungeon.
Edit: I will also note, that I ran Clearing Castle Caldwell as the very first adventure for the above-mentioned "College Game." A party of four 2nd Edition characters (Ranger, Fighter, Fighter, Cleric) with experience got through both levels with some bumps and bruises, but still ran around like crazed little girls when they opened the door to the courtyard.