I have heard of several people on other forums, and on one erowid trip report describing it as being used because it is able to "break through" methadone treatment and still deliver a high probably because of NMDA antagonism. I have also heard that several reports referring to it as the most euphoric opiate because of its intense rush upon administration. However, the current pharmacutical preparations prescribed in sweden use a salt which dramatically reduces bioavailability increasing the required dose, and reducing euphoria. The few people who reported using it prior to the change in preparation that I was able to find described it as the having a greater rush than any other opiate.
Psychedelic Jay- I do not think you are at risk of a dissociative "hole" because its painkilling properties come from the addative combination of NMDA and opiate agonism. This is why it takes less than a euphoric dose to provide pain relief while the NMDA antagonism is comparatively weak against mu opiod action. I also think that this means that it may not be as much of an overdose risk as may be expected because NMDA antagonism is virtually impossible to OD on, and so at euphoric doses you would feel a stronger high, but less risk of overdose because less of the effect comes from opiate receptors, and thus you are less likely to OD.