Agreed. I think it's pretty hard to not know what it's like to feel stupid at least some of the time. Even most geniuses are 'stupid' in at least one area. Most of us have felt the sting of seeing a colleague, friend, or stranger pick up a challenging topic with enviable ease.

The more interesting question is, what it's like to feel that way all the time. I suppose we all choose to associate mostly with those of approximately equal intelligence to ourselves, so we're probably mostly oblivious to our relative intelligence.