Champix is truly amazing but can come with some side affects if the dosage is not tailored to your habit.
The thing to know about Champix is that there is the standard,generic dosage and then there's your required dosage (determined by how much you smoke per day and how long you've been smoking). I personally have smoked about 8 a day for 4 years and after 3 days on the 1/4 dosage (first two weeks you take a 1/4 dosage to be sure your body doesnt freak out) I had stopped smoking completely and didn't have a craving or a side affect to speak of.
Stupidly, I then followed the instructions on the pack and jumped onto a full dose which, after 2 months, started to give me heart palpatations and head pulses. This was quite disconcerting and caused me to stop taking the drug. I think I probably could have just gone without from this point but I sort of convinced myself at the time that I needed Champix in order not to smoke and subsequently went back to smoking.
This Friday I'm starting champix again but will be taking 1/4 doses per day until they are used up and then 1 full dose tablet every 2-3 days. Just so you know, it was a week after I had stopped taking Champix that I had a craving for a cigarette which shows how it builds up in your system.
So I guess if you smoke a 25 pack a day and have been doing so for a number of years, the standard dosage would be good and you would probably not stop smoking completely until the start of the full dosage (as the instructions state). If you smoke less then you should definitely moderate the full doses and up it as required.
This is by no means to be taken as medical advice, it is purely my take on Champix.