I would say it to people I already know or have met before, but I doubt there's a rule. To me, the question implies an real interest (at least more than "How are you?") on the part of the speaker.

Thank you for your answer!

Does anyone else have any idea?

To my English ears, "How is life treating you?" seems quite inquisitive, so I wouldn't say it to anyone I didn't know very well, and, on reflection, I don't think I have ever said it to anyone.

However, "How has life been treating you" is a question that frequently appears in "Lost Touch" columns in the newspaper and on the TV (Teletext). When people are trying to get in touch after a long time, this seem to be one of the standard ways of writing - e.g.
"Seeking ABC, worked/at school/lived XYZ in 1970. Love to get in touch/remember DEF. How has life been treating you?

If someone would ask you "how is life treating you?" what are you expected to answer. Should you only answer as if asked how are you or are you normally expected to say more?

I always thought this expression was an Aussie thing to say. Probably b/c I first heard it Down Under.

I felt it was a more personal way to say "How are you?". So I always answered it that way. "Not too bad" seemed to be an appropriate answer! ;o)

Not too bad ist doch einfach ein nichtssagender Ausdruck, oder? Ich glaube schon, dass da vom Gegenueber mehr erwartet wird als Antwort.

Also, ich bin kein native speaker, aber wenn mich jemand gefragt hat "how is life treating you?" dann war es immer eine freundschaftlich augenzwinkernde und aufmunternd gemeinte Anfrage zu einem Zeitpunkt wo es grad NICHT so gut ging. Also, wenn man sich grad tierisch ueber etwas aergert, im Stress ist und bis morgen noch ne Nachtschicht einlegen muss, um ne deadline zu schaffen z.B. Von jemandem, den ich zum ersten Mal treffe, wuerde ich die Frage nicht erwarten.

Hello,
To my english ears, "how is life treating you?" requires a more detailed response. I have a friend who always asks me this, and she expects a detailed answer. If I just answer, "oh, ok," she presses me for more details. So, from my experience, "How is life treating you?" is something you ask a friend. "How are you?" is something you can say to anyone.

"How is life treating you?" is NOT appropriate on formal occasions, e.g. boss, company visitors, cashier at the supermarket. I would expect it only between friends, and even then more likely with friends who you perhaps don't see so often, so that you are indeed interested in what's been going on in their life since the last time you saw them.