I personally have had no side effects either time I've used them. The first time around, I took them for 3 weeks, and I was off smoking.
This time, I'm finishing up my 2nd week, haven't had a single puff all week and I'm doing great with it. I've even been hanging out with friends who smoke, and I don't even want one. I'm going to take them 1 more week, then quit the Chantix, since the nicotine should be out of my system by then.
I have never lost my desire for sex while on these pills. Actually, I've had a hell of a lot more energy since I've been on them.
I have definitely not lost my desire for food
The first time I quit, I gained about 15 lbs in 2 months. I eat out for almost every single one of my meals. This time I have changed it up a bit. Since day 1 of Chantix, I've cooked every meal, and I eat a handful of pretzels a couple times a day between meals. This makes me less "starved" feeling, which makes me eat less. It also brings up your metabolism a little by having smaller snacks and meals throughout the day.
I've already lost 2 pounds this week by doing this, and I haven't even been to the gym. I've been too busy the past couple months with full time school / full time work. I feel better than I have in a while already. I don't think I'll ever pick smoking back up again, no matter how stressed I am.