I have. Quitting smoking is a little bit harder because smoking is a binary state. You either smoke or you don't. Where as with gaining the lost weight, it doesn't happen over night and thus easier to maintain. At any rate both are long time struggles that are hard to hack.

James' blog post about losing weight was somewhat of an inspiration and validation of the diet I was following at the time and its effectiveness. One advice that I still follow is when I'm pressured into food that may contain flour or sugar I just say I'm allergic which works great.

I've lost about 120lb from high school (320->200, I'm 6'4 and 28 now). I also currently smoke. I've quit smoking for about a year and a half, probably 3 of the last 8 years combined have been not smoking. Smoking is harder for me, as it not only requires a pretty vast social adjustment (avoid smokers at first, pass each and every willpower check), and stress brings it out of me like crazy. You need to have something else to replace the quick shots of dopamine you can get any time you want. I am currently working on new habits, and have my diet under control again after gaining some of that weight back. Next is exercise. One thing at a time.

I went from ~280lbs to 170lbs a few years ago. Still ~185lbs. But smoking I fear will eventually kill me :) I've cut back recently supplementing with a vaporizer. Not sure it could replace cigarettes, but they're impressive enough it's possible.

For me, either one is easy if it coincides with a significant change in daily life (moving, new job, new school, etc).

not sure a massive % but I lost 15 kg out of 94 some years back. Not very hard if you do it the slow way (i.e. in a few months, with a dietician and exercise) but for some people it's a lot harder.

I also quit smoking a year ago after more than a decade. It was definitely harder, but it's not impossible.