For the last, say, forever and a day or so, people have uplifted, encouraged, and congratulated each other with a generous blessing: “Good for you!”
For whatever God-pounding reason, I now hear people saying: “Good on you.”
Fuck your Good On You.
Keep it offa me.
That’s all.
Sorry, I meant to say ‘good for you’, I said it that way on accident.
Apologies are nice. Good on you.
I believe it’s an Australian expression. An Aussie friend of mine used to say it all the time, and I think one of the sharks in Finding Nemo said it too.
Bless your heart.
It’s an English expression too. I think since the world is getting smaller and so many are conversing with people from other countries on the internet, we’re starting to use expressions we’ve rarely used before. But I may just be trying to take the piss out of you.
Between this thread and the ones about “No problem”, I think it’s safe to say that there’s no expression, however innocuous and well meant, that doesn’t inspire someone to rage.
It’s all good.
How dare they not say: Good on ye. Languages should never change.
Let’s go to the quarry and throw stuff down there!
That was the very first time that made me laugh, Bo. Don’t do it again.
No worries, mate.
It’s common here, yes. “Good for you” sounds sarcastic and patronising to me.
Put for down as one who shares the OP’s rage. Unless spoken with an Australian accent, I get all stabby when I hear that phrase. It does seem to be getting more popular. Perhaps there will come a time when it doesn’t sound ridiculous. Perhaps.
(I know that New Yorkers say “stand on line” instead of “stand in line.” Even after living in NYC for three years, my ears never adjusted to it.)
I like “good on ya.” It’s spunky and friendly. I say lets use it more frequenty. Og knows we need more friendly.
Wait until someone cracks out the old Onya Sonja!!!
Veggie.
(sits back and waits for the howls of anguish). 
Onya, OP.
“Good on you” is just Britspeak. Or at least Aussie speak, in which case I usually hear it as “Good on ya.” Hear it all the time over here.