Highest dose was around 500-600mg, around 2 bottles of cough syrup (only medical ingredient being Dextromorphan Hydrobromide 15/ml - no paracetemol or other additives).
There's ample reason why it's not more popular; it's just not very fun. Whereas LSD and psilocybin mushrooms and mescaline and other psychedelics have a more erotic, fun, energized feel where things are colorful and drippy and creatively, DXM and Ketamine have always struck me as the other side of that coin: the dead and black pit of stagnant dissociative delirium.
Yeah, it at a high enough dose it gets "trippy", but not in the normal, happy-go-lucky "holy shit I'm so ALIVE" acid peaks. Rather than a bubbling perception of space and time littered with interlacing patterns, it's just kinda a claustrophobic, drunk, disorted sense of dimension with the usual visual tracers, but in a much more "sickly" way.
It's death tripping. It's a zombie buzz. Flushed face, cotton mouth, indigestion (from all the syrup), slurred speech, zoned out, sensory-motor disconnect, blank thousand yard stare and vertigo and confusion. The whole fucking mess it makes of you is just yucky.
No hysterical public service, "it'll fuck you up for GOOD" (unless you make it a serious habit), but Christ, give it a pass...