I used Chantix and quit. It's not a substitute drug, you only take it for a few months. I only took it for 2 months and haven't had a smoke in almost a year. I was a pack a day smoker for quite a few years before that. Currently, Chantix is going through trials for combating cocaine and alcohol dependency issues at U Penn and the short-term success rates are looking good. People who have taken it before probably know why.
The behavioral and suicidal issues are an entirely different and far uglier situation. Many people with mental disorders smoke as a way of self medicating. Some of the same factors that predispose someone to nicotine addiction (and we now know that this is a genetic predisposition) also predispose individuals to certain kinds of mental disease; schizophrenia, bipolar disorder, and borderline personality disorder, for example. When that form of behavioral self-medication is removed from the individuals daily ritual, it begins an unpinning process. The reason why people harm themselves or have those thoughts is because they've successfully quit, not because of the drug itself which works on only a few highly specific neurotransmitters.
The actual research on Chantix shows not a causal pattern, as laypeople, bloggers, and other hack journalists have interpreted and stated, but rather a pattern that is far more telling of the number of individuals who self medicate with cigarettes. Because most cigarette smokers also abuse other chemicals (alcohol and caffeine especially) and Chantix works on the same neurotransmitters that would bring feelings of "peace" or "joy" to the body from imbibing other chemicals, individuals on Chantix often begin to find little to no pleasure from other substances they've come to self-medicate with. This, for obvious reasons, can cause serious problems. The distinction may seem slight, but scientifically it is rather important and is one of the main reasons why varenicline hasn't been taken off the shelves.