Zyban is actually another marketing name for Welbutrin, an anti-depressant. They found out that a cool side effect of the drug was that it makes cigs taste like absolute dog crap. Believe me, after a few days you take a drag and have to fight the gag reflex. Anyway, the FDA approved it as a stop-smoking drug. It makes you happy while you quit.
I had great success with it (doctor prescribed, of course), others have not. You have to be careful about tapering off the drug when you quit, or you turn into a real a$$hole...
How long had you smoked before you quit? I'd really love to quit, but I've been smoking for about 15 years and think it'd be harder than heck!