I don't celebrate Christmas - just a decision my wife and I have made. It's pretty sweet not having to hang out in malls and buy useless shit for family you see once a year. We're atheist so there's really no point to celebrating the holiday (for traditional reasons that is - I do enjoy spending time with family.)
Saying that, if someone wishes me a "Merry Christmas", I'll reply in kind because it's respectful. If someone wishes me a "Happy Holiday", I'll reply in kind because it's respectful.