I pack my lunch sometimes, though not often since I work from home currently. But I still have to do so from time to time for other reasons.
Generally the best things I think to pack are portables like bananas, apples, boiled eggs, nuts, yogurt, pieces of cheese, crackers and/or various sandwiches. I also used to carry sardines at one point, I went through a phase.
Of course, many companies make it super easy for you to carry junk food like candy, cookies, and potato chips in individual serving packs, but I tend to avoid that most of the time.
When I worked in a place with a microwave and/or a fridge, or when I took things to school with me when I was in college, I was more likely to take microwavable meals (soup, pasta, lean cuisine, etc.) or left-overs.
I've never been devoted enough to carry things like fancy little bento boxes.
The most common problem with doing this is that sometimes you can be tempted to spend money even though you brought your lunch because fresher food or restaurant food looks more interesting.
"Sentiment without action is the ruin of the soul." - Edward Abbey
I usually do. It's usually simple things. I just made my peanut butter and blueberry jam sandwiches for tomorrow and cut up some strawberries. I'll usually take leftovers if I have them. I do run into the 'everything else looks better' scenario, but I am normally so busy that I don't have a break, so it's just easier all around.. and I save money and it keeps my weight in check too.
"It is not length of life, but depth of life." ~ Ralph Waldo Emerson
I almost always pack my lunch because it's cheaper and it's a lot healthier than anything that I could get for a quick meal on the road. I pack my lunch the night before and prep for the whole week on Sunday evening (which is about the peak of my organizational skills!). I can't take anything that needs to be heated up since I usually don't have access to a microwave. I usually take a pasta salad, fruit, string cheese, a vegetable and some almonds... I tend not to get bored with eating about the same thing every day for lunch for some reason
Human beings make life so interesting. Do you know that in a universe so full of wonders, they have managed to invent boredom? -Terry Pratchett