Festivus is one of the many great things that came out of Seinfeld. I think some people may celebrate as an homage to Seinfeld. Others may celebrate it to get away from the commercialism of Christmas. It's hard to say if anyone "seriously" celebrates the made up holiday, but it is possible.
I don't celebrate, but I know there are extreme fans of the show. These fans probably celebrate it. And who cares if they do? It's a free country. As long as they aren't hurting anyone It's all good. Let them keep doing what they are doing. It's probably making the world a better place anyways.
Festivus, by its very nature, can not be seriously celebrated. The entire premise of Festivus, including its obviously comedic rituals such as the "Airing of Grievances," is a joke. While those particularly committed to the joke may celebrate it, and may do so with gusto, by no stretch of the imagination can it be labeled a "serious" celebration, on the same level as sincere celebrations of other holidays.
I don't know of anyone who celebrates Festivus. I've never even heard of it. It sounds like a play on the winter solstice, but living in the bible belt where people get offended if you don't say Merry Christmas, I don't see it catching on and good luck getting rights for it at work.