I also have a special place in my heart for dextromethorphan. I regard it as one of the most psychically helpful chemicals I've tried (right up there with LSD and the 4-OH-tryptamines). Dextromethorphan seems to trigger emotions in me that I have never felt sans chemistry, such as nostalgia and sentimentality. It is quite sad that this amazing tool for psychic introspection has become associated only with lame-ass 14-year-old stoners looking to kill a boring night by getting 'fuXor'd uP on c0ld m3ds.'
But there is so much more to dextromethorphan than just the NMDA recpetor. I think that the sigma receptor activity plays a large role as well. With ketamine, the same story applies--there is more going on than just NMDA receptor antagonism. And obviously, with phencyclide, there is a potent stimulant effect (PCP will fully substitute for cocaine in experimental animals), mediated by both the DAT and the dopamine D2 receptor, in addition to the intense dissociative effect. As a aside, this locomotor stimulation might be why PCP is known for producing such odd, violent behavior: think about how many cokeheads act like paranoid, arrogant twats after snorting a few lines...now couple that with total dissociation. Back to the original point, since MK-801 is pretty much completely selective for the NMDA-receptor Mg++ site, it might be a powerful dissociative, but I doubt it would have any of the positive effects of either ketamine or dextromethorphan.
Edit: I'll throw in a little epilogue to my dextromethorphan bit above. I concur regarding the intensity of the dextromethorphan next-day hangover/afterglow. In addition to a classic alochol-like hangover feeling (lethargy, fogginess), the day after dextromethorphan, an eerie emotionality is cast over everything. For me, that day is like experiencing what it must be like to have one's period, minus all the physical pain (I'm a dude, if that wasn't already obvious).