Used it extensively
Used Comeliness and still wouldn't mind using it in 3.5e to explain physical beauty and attractiveness.
Oddly enough the Cavalier and Barbarian were never really used by PCs as we were rigid in enforcing a code of conduct on the Cavalier and not letting Barbarians really work well with Magic-Users.
Weapon Specialization was used extensively
Demihuman advancement made the handful of Elf F/MU/Th characters more viable for high level play.
Drow were always NPC
Spells were great but some spells like Chromatic Orb seemed a bit more powerful than Player Handbook spells.
Alternative Creation methods based on class were used (You could actually reasonably expect to create a Paladin for a game without using some ungodly roll up 30 sets of stats and choose the best one

)
Used the social class rolls pretty extensively
The demihuman dieties was useful in supplementing the core ones from Dieties and Demigods but no one ever played a cleric of Baghtru (although there were some NPCs that were)
The depictions of polearms was actually pretty damned useful in differentiating the various polearms and are arguably better than the depictions that followed.
Overall I'd say it became a pretty critical part of my 1e game.