The phrase can be said approvingly, but most of the time I hear it when adults are talking to children (after they've just accomplished something).
The phrase is used sarcastically usually when someone hears another person boast about something that they achieved.
Example:
Bob- "Hey, I know you no one has asked you yet, but five girls just asked me out to prom!"
Jake- (sarcastically) "Good for you!"
Most kids/teenagers will say it sarcastically, though honestly people mean it both ways all the time. Just be careful how you say it, because if you do actually mean it approvingly, and you come off as being sarcastic you can hurt someone's feelings.
Hopefully that makes sense.