If you're charismatic enough you can get away with saying how you really feel and you'll find that you make friends more quickly. I remember there was this time in the midst of a relationship ending and my father dying and after years of dealing with a painful medical condition I responded with "it's just another shitty day" with a rueful smile. Instant friends with the cafe worker. When things finally started to turn around for me and I said "things are great!" he was really happy for me.

It's a matter of reading the person and situation. Sometimes if the person asking the question is in a context where I can tell they don't want an honest answer I just say "oh, how are you?" I'd rather evade the question than to say "I'm fine" when things are shitty.