Yes, I choose to say Merry Christmas. Yes, I believe that Christmas is about the birth of Christ and that should be the focus. However, (and I know that some reading this will just be appalled by what I am about to say) I do not expect that those that don't believe the same way that I do to say it. Some choose to say Merry Christmas back, some choose to say "Happy Holidays" I don't get upset by it anymore. I also have not had anybody get offended when I do say "Merry Christmas" when they say "Happy Holidays" first. Why is it that we as Christians in everything else, have been taught and believe that we can not expect a non believer to walk and act like a believer when they are not, except in this instance. Christians get so outraged, some to the point as if when somebody says "Happy Holidays" it is like they started sacrificing to Satan by just saying it. I used to be one that got outraged. I have come to the point to realize that me getting outraged at those that don't say it, when they don't celebrate the way I do is not going to make them believe it.
However, I do think that we as Christians should stand for what we believe and not just allow "the world" to take Jesus out of it. We should be able to buy Merry Christmas decorations, set up nativities, say "Merry Christmas" without fear of offending somebody, buy a " Christmas Tree" (I mean, seriously, who else calls it a "Holiday tree" but the retailers who sell it that way)
Here are some links on the history of why Christmas is celebrated on the 25th and where Christmas trees came from. Christmas History and Christmas tree history.
I believe that if we stand for what we believe and do so with love to others that don't, we can be more effective then going around with a chip on our shoulder getting angry over what somebody says then looking at their heart with understanding and talking to them logically about why we believe what we believe. In the end it is not about what gets said but how we can tell and show others what Jesus came for so they can have their eternal destiny changed.