DXM, PCP and ketamine have all been shown to act on NMDA receptors in the brain (among others). They all have anaesthetic effects, or make the user feel as if their mind has been disconnected from the body.
The effects of DXM are seperated into dose-dependant plateaus or levels of effects. As the dose increases the effects change from feelings of drunkeness to full blown dissociation and hallucinations in the "4th Plateau".

Im not into DXM very much, and the strongest experience I have had was probably in the 3rd plateau after having chugged a bottle of robitussin. I felt pretty numb and extremely uncoordinated, loosing sense of space and time. I don't remember hallucinating, or anything else for that matter.

If you want more information on DXM than you'll ever need to know there is a whole FAQ on it, including in-depth descriptions of the plateaus. It can be found here: http://www.erowid.org/chemicals/dxm/faq/dxm_faq.shtml