Just saw this thread. This is also my most debilitating symptom. I often find it hard to describe to people - frequently resort to 'stoned without drugs minus the happy feelings.' Although I don't think that is quite accurate.
I have different versions of it - one of them is pea soup brain fog. At these times it literally feels like something has gathered in my brain - like I can locate it at the centre of my forehead - glue or fog or treacle and my thoughts have to battle their way through it. It impacts on memory, quite literally from moment to moment - not just distant memory but what I was about to say/do, the next bit of a sentence. It makes any kind of analytical thinking very hard. Straightforward questions are ok like - have you read this book? However a question like - 'what do you think of this book?' would leave me struggling to pull my thoughts together. In fact it impacts on my memory so hugely that when I am like that I can feel lost and a bit personality less. It's made me realise how much of who we are is dependent upon our memories, what has happened to us, the opinions we have formed etc... When I am like this I feel more thing than person.
The other version is even harder to describe. A neurologist once said to me that 'migraine slows the brain, ' and that would be one description - a slowing of thought. Or rather than being stoned that I have taken a slightly more hallucinogenic drug which brings on a dreamy, surreal, floaty brained quality. At these times I feel like I can't catch my thoughts, like they are floating away from me. My brain feels light in a bad way. This is often accompanied by extreme light sensitivity and a sensation visually that things are simultanously too near and too far away. So the whole experience is, I am guessing, akin to an acid trip. I have never taken acid so I don't know for sure.
I am a screenwriter, so this has been a major battle with regard to work. Although I had the brain fog for the first two years of illness, I have had long periods of remission from it, which has allowed me to work again, although I have the brain fog currently.