It's definately art for me. While i sell most of my customs, i do hold onto a few of them that i wouldnt sell for any amount of money. For me, i get a much bigger kick from knowing that someone around the world has something I MADE in their collection and enjoys it more than i ever would (having it sit on a shelf, unseen, and probably dusty.)
I suppose in a way its also a hobby though, i don't support myself off customs so itd definately only be a "hobby" to me. But anyone who makes a custom figure and im not talking a parts swap or a repaint, the second you start sculpting or doing extensive work on a figure, it becomes art, for better or worse, lol.