I have experienced derealization, a form of dissociation, for 30 years. Besides a severe depression I'd say the derealization is my worst symptom. For me, it feels like I am looking at everything through a glass wall. I am connected just fine to myself; it's that there's something odd between me and the visual world. I (and some pdocs) have suspected I have some type of epilepsy, though.

You can take a look at minddisorders.com (under depersonalization) or dpselfhelp.com for a lot of info on depersonalization.