The two times a year I do it, here's my reasoning:
I give one huge problem that has 20something parts. (I teach Geometry. Once it's a huge circle, the other time it's a huge triangle containing parallel lines, altitudes, angle bisectors, and anything else I can think of inside it.)
I tell the kids that tomorrow I'm going to ask for a consensus on the answers. Then I shut the doors to keep the noise from bothering other classes, and let them work. Some kids form groups, others are comfortable working on their own. I walk around the room from one group to another, dropping hints if needed to jumpstart anyone who needs it.
But when someone gets stuck, he or she can simply ask someone for help. Some kids are very hesitant to ask too many questions; they're afraid of looking stupid. Yet they're less hesitant to ask their good friends the same question. I've also found that even the strong kids learn a lot by explaining their work to their friends. They say the very best way to learn something is to teach it, and I see that when I do this particular assignment. I always catch a lot of "aha!" moments.
Inevitably, they get at least a 9/10 on the quiz; the vast majority of the time they get all the answers correct. And it helps break up the rythym of class. After the first time, they always ask whether I have any more "mega-problems" and are happy to learn that there's another on the horizon.
Oh, and for the record, I would like to offer a bit of support for all those kids who don't earn the A's. Sure, some of the kids are the top of my classes are the ones with the strong work effort... some of them. Contrary to popular belief, not all the "industrious" kids are the ones with the highest grades. Some of my most industrious kids work their tails off for a B, or even a C. I've taught some incredibly gifted, incredibly lazy kids as well. For some,math is easy.... they see the logic and just don't have to work as hard to attain those high grades. Not every kid with high grades is a good student. And some of those A students are in it only for the grades; maintaining that precious "4.0 average" is all they care about. Some will happily cheat to get those 4.0's if they think they can get away with it. Sometimes Honors classes can be tough to teach. Not because of the material, but because of the overwhelming concern about grades, on the part of the kids and their parents. Sometimes the Honors kids put those high grades above all else, while their "average" classmates exhibit a real love of learning.
Some of the most industrious, hard working kids I've ever met have been C students. Ask any teacher who has been in the profession for a while; the correllation between work ethic, or decency as a human being, and grades isn't always as high as some might think. Good kids, good work ethics, good study habits come with a pretty wide variety of GPA's. Please don't deny your "A student" sons and daughters the opportunity to learn from some of those hard working C students; you might be surprised at the lessons some of those kids can teach. Some of them put in many, many times the number of hours per night their brighter classmates put in. And they persevere, even when the results of those hours of studying don't land them on the honor roll.
I'm kind of stunned at some of the comments I've seen on this thread about kids who don't achieve those top grades, and the names they've been called. If only you could spend some time in my classes, you would come to know some incredibly wonderful kids who don't happen to be at the top of their classes. But I would be thrilled if my own kids brought these kids home as friends
While I'm not a fan of group work the way some here have described it, I'm not sure I would label my colleagues as "mean" simply because they don't share my point of view.