that the upside of the "magic" of Christmas as a kid outweighs any negative effects of lying to our kids. I played along until they were old enough to ask me directly if Santa was real - that is where I drew the line. They each believed until they were about 8-10 years old.
I also drew the line at Elf on the Shelf - no way was I going along with that. My wife did it for a year or two before she realized what she had gotten herself into.
I know I'm the board freak.