For me it's got to be the derealization feeling as well. Just the general "weirdness" feeling, I suppose. Like Spriggy, I can have it for long periods. I just don't feel "myself" when it gets like that - and that's always pretty disconcerting.
And [b]every[/b]time it comes on, I convince myself that I'm developing schizophrenia (even though I know I'm not), which just makes me feel worse.