Ugh, that's sad. He outlived his prognosis by about half a century, but I'd really just come to expect him to keep living. His tragic circumstances really served to enhance his public persona, and the way he managed to remain active within the scientific community and maintain his sense of humor was really inspiring. As a kid he seemed almost more like a mythological being or something of that sort when I'd see him on Star Trek and elsewhere.
I'll always love him for how he took the time to simplify and explain extraordinarily complicated concepts in a way that we mere laymen could understand and appreciate. Very sad to see him go.