I am another one who failed on Chantix. Tried once in Nov of 2007 when it was first released. Did well through March, and then I let stress put me right back smoking again. It did make me quite sick, but I was so tired of being chained to analogs that I didn't care.
The worst came on my second attempt in mid-2008. No clue as to why, but this time, it didn't make me physically ill -- it completely and totally changed my personality, and not for the good. I am single, so live alone -- making these kinds of changes impossible to see as it normally takes someone else to notice them. My BF at the time and my Boss were the two to pick up on it. It almost cost me my job.
I am normally very pleasant and class clown-ish and apparently I became bitter (!!) and nasty (!!!!). That has to be the most frightening thing I have ever experienced. Scary because it sounded like the boss was describing someone else, someone very unpleasant to be around -- but he was talking to me about me and I didn't see it.
It took 4 months working with a homeopathic doctor to get this poison out of my system and back to being myself. The boss had no idea I was on meds. Once he found out it went from "WTH is wrong with you lately" to "what do you need from us to help", so it worked out well in the end, but still.
I wouldn't go near this drug again even with a gun held to my head.