Since ALS is a negative diagnosis disease we really don't know how many diseases or sub types there are all being called "ALS".

Some people live 6 months or less, others live 8+ years.

If you get on a vent and feeding tube you can significantly extend your life.

ALS can impact brain function in strange ways.  It can cause memory loss.  It can cause a heightened startle reflex.  It can cause you to laugh or cry at inappropriate times.  It doesn't have to do any of these things.  It doesn't lower IQ, although people with a high IQ might have a slightly better chance at getting it.  Of course after around 24 everyone's fluid IQ goes down, so Stephen's probably did too.