i did it a bunch of times last year. either by drinking tussin or eventually just capping pure DXM powder. it used to be pretty cool, but it's just too unpredictable. your trip depends greatly on the atmosphere. for me, i gotta be outside to enjoy it.
DXM is a dissociative, similar to ketamine or pcp. it's said to be to be between them in harmfulness.
if you even think about doing this, know what you're doing. do NOT consume anything with acetaminophen in it. most kinds of robitussin and coricidin have this in it. at the doses required to trip, you will be consuming a LOT of acetaminophen, and this can kill. if you're going to do it, check the ingredients and use something that has ONLY DXM in it. most medications have pseudoephedrine in as well, and that's no good at high doses either. your heart will be beating like crazy and you will not be able to sleep.
i've had out of body experiences on it and experienced some other really surreal shit. it can be incredible, but very different than LSD or shrooms. be aware that DXM is an anesthetic, so if you go out and hurt yourself, you probably won't realize it until the next day. at higher doses, the closed eye visuals can be intense, and once you close your eyes you're in a different world. but just open your eyes and you're back.