This brings me to the most relevant new information about DXM: Olney's findings of NMDA Antagonist Neurotoxicity (NAN). There is great debate right now whether NAN is relevant at recreational doses or not. In animals, the dosage required to induce NAN is far in excess of the anaesthetic dose, and humans typically take sub-anaesthetic doses of dissociatives. On the other hand, there may be danger with long-term use at considerably lower dosages that the animal models do not show.
The data from human experiences are hard to interpret. Many heavy PCP users suffer obvious cognitive and motor impairment; however, PCP has neurotoxic effects (in particular in the cerebellum) not shared by other dissociatives including DXM or ketamine. Ketamine is probably a better approximation of DXM, but very few people have done large amounts of ketamine for long periods of time. A notable exception is John Lilly, who is a bit of a nut, but was probably a bit of a nut before doing ketamine, and (at least the last time I checked) he doesn't seem to suffer from cognitive impairment.
There are a few DXM users who have suffered long-term consequences. Out of approximately five hundred current and former DXM users I have heard from, three have suffered lasting cognitive impairment. Additionally, there is one published paper on cognitive impairment from chronic DXM use, although the author suggests an underlying temporal lobe seizure disorder. DXM has been shown to increase the frequency of complex partial seizures, and it's possible that it is the seizures, and not the DXM itself, which is causing problems. Unfortunately, it has also been suggested temporal lobe epileptics may also be more susceptible to dissociative addiction. Hopefully much of this will be resolved in the next few years.
Until then, my official recommendation is not to use DXM at all. Since I know this isn't likely to happen, my own personal belief is that DXM is probably pretty safe when used occasionally (e.g., once or twice a month) at the lower plateaus, and rarely (e.g., once or twice a year) at the higher plateaus. I have yet to hear from anyone who used DXM with this or less frequency who has suffered any impairment, temporary or permanent. Actually, to be technically correct, nobody using it once a week for less than six months has ever seemed to have problems, but it's always best to keep a wide safety margin.