I don't find it offensive (but I'm Christian, so it wouldn't bother me in the first place). That being said, I think everyone should be able to say whatever they want, as long as they say it with sincerity. If my Jewish friends wished me a Happy Hanukkah, I certainly wouldn't be offended, and wish them the same in return. I have many family and friends that are of a variety of religions, and if they sent me their best wishes for their holidays, I'd do the same. I don't find it offensive as long as it is sincere. Say what you are going to say, but mean it.
Besides, the whole idea of saying "Happy Holidays" instead
of "Merry Christmas" kind of baffles me. It was always my understanding that "Happy Holidays" was a shortened form of "Merry Christmas and have a happy New Year."