I definitely believe in his spirit. He existed way long ago, and I believe that he watches over those in need and around christmas helps inspire people to be generous etc. I just find that that's harder every year as our greed and avarice grows ever bigger
I don't believe there's a real santa with elves and reindeers that let him go trough the air..
But the thought is always nice and the happiness it brings to people is even better. Who doesn't like it to be with his friends/familiy around the christmas tree with a lot of presents and stuff to eat =)
I started to not believe in Santa when I realized that if Santa were real, everyone would say his superhuman abilities and flying reindeer were "roof" of God or some higher power. I was 10.