Calories in vs calories out: Yes
Weightloss a matter of willpower: I agree with reservations
There is certainly a certain amount of willpower involved, but most people are undereducated on what it takes. They try what works for others (or they think they do) and after not getting much success, give up. For myself, I am male, 6'3" and was 155lbs in high school. I stayed around the same weight in college, then joined the Navy. Coming out of boot camp I weight 180lbs, and it wasn't fat that I gained in those 3 months of pushups. After the Navy I stayed around the same weight for awhile, but my desk job started getting to me. I got up to 215 pounds and was getting some nicely defined love handles.
I had already been eating much better than most of my coworkers (earning the some chide remarks about my eating a bag of frozen broccoli everyday for lunch, along with a frozen meal and some a piece of fruit). Oh, and I had been eating 2 packets of instant oatmeal for breakfast (flavored variety). So when I decided to start dieting I switched out the flavored oatmeal for plain, and would flavor it with unsweetened apple sauce and cinnamon. I also switched to the leaner frozen meals for lunch, tried eating less at dinner, and basically other "diet" stuff. I went down in wait, but couldn't budge past 195lbs. I would try eating less, but could only keep that up for so long as I was hungry all the time. I knew I could weigh less, and that I should weigh less (considering my activity level). I then discovered The Primal Blueprint (which eventually led me to MMM) and started adopting some of its principles. I increased my fat intake and cut out the grains and cut down on carbs in general. I became "fat adapted" and so no longer got headaches when going for long periods without eating. I was able to start skipping breakfast and actually felt better for it (didn't eat till 2 p.m. today and really only ate because I noticed the time and realized that I hadn't eaten yet). My weight got down to 175 lbs. I started doing some heavy lifting and as I've gotten stronger (and bigger) my weight sits around 178-182 lbs depending on the days.
Measuring calories now, I get about equal (usually less) than I did when I was starving myself to try to drop weight. My willpower is no greater than it was before. My education is greater. My understanding of my body is greater. But my food is more satisfying now, so I don't need to eat as much of it. Which ultimately ends up being fewer calories in than out.
Though yes, sometimes it can be hard when I'm with extended family and they have some delicious looking food that I know I shouldn't have. Sometimes I will partake just a little. And sometimes that will remind me that while it looks pretty, I don't miss the old food (or how it makes me feel). Other times it pure heaven and at those times it does take willpower (though sometimes I will indulge and just make sure I don't do something like that again for at least a couple of months).