I have a friend whose children were literally furious with her when they worked out that she'd been lying about Santa (which they immediately extrapolated to the tooth fairy, Easter bunny and so on).
Having young children myself I've thought a bunch about it. I'm not wild about the lying but when you see the enjoyment they get out of it, look back at how you felt about it when you were young (where deception doesn't rank highly amongst my recollections) and think about some of the other consequences (them being the only child in the playground who doesn't believe) generally you come down on the side of the lie.
My compromise (if you can call it that) is that I won't be one of those parents who strings it out when they seriously start questioning it. Aside form anything else at that point it seems like a great opportunity to ask them to exercise their reasoning skills to talk through why they're questioning it.
I'll admit it's nice, but at the same time it's delusionment.