Very rarely in its original sense anymore... if you read a few books and some older text then it's not so unfamiliar in it's original meaning. I like the original, happy abandon of it's first meaning.
I sometimes use it jokingly, but you need to know the person you're speaking to!
Actually, thinking about it, the term 'Gay' meaning homosexual, derives from it's original use anyway. Someone who was gay would be bright and colourful, extroverted, the life and soul of the party etc. So it makes sense.