too much info

When is sharing too much?
Recently, I overheard a phone conversation between two aquantinces (sp?). Person A asked Person B how they were. Standard small-talk. Person B responded with a drawn-out, jibber jabber of "well, not very good, sickly, lots going on, don't want to bore you with it, don't want to get into it."
I'm certain there was an awkward not-sure-how-to-respond moment for Person A.
When asked how you are by someone you are not at a friend status with (say a work client), what is the appropriate level of response?
If you are having a bad day, do you say so? Or do you politely respond "I'm fine, how are you?"
If you are sick, do you say so? with how much detail?
Personally, I feel that this is a less-is-more situation. I don't feel the need to tell anyone that I'm having a bad day, unless it is someone I want to discuss my bad day with. I was put off by the overheard conversation. I think it's rude to allude to "badness" or profound illness to a semi-stranger. Internet, what's your thoughts? Are there boundaries?