How many people are affected by input lag?
Everyone. Whether they notice it or not.
Do people get used to it and are able to compensate for it?
Yes, people can whether consciously or subconsciously, but not everyone will. Depends on how sensitive you are to such things.
Do laptops and netbooks have this problem?
Is there any way to tell how much input lag a monitor has before buying it?
Only if a review of the monitor includes such tests.
Does it really make first person shooter games unplayable?
For the average hardcore gamer, no or very few will complain to a large degree. If you are a dedicate competitive gamer (meaning it is basically your living), then I would have to say yes because any lag at all can mean elimination from the tournament which means you walk away will very little prize money, if any. To be a competitive play you need very fast reflexes (which in itself is a form of input lag), more than just the mere hardcore gamer.