I really like this product. I also use it far less than I should, which is really my own fault. It's well built, other than the slightly cheap feeling lids on the upper two containers, it keeps heat reasonably well, and it comes with a handy pouch and even a spork.
As others have mentioned it stacks four containers, with the bottom one being fairly small and somewhat spill resistant. Above that is the largest container with an insulated lid. That lid is in no way airtight so you'd better put something in that container that cannot leak. That lid also marks the barrier of insulation - the two tupperware-like containers will catch a bit of radiant heat so you may find whatever's in the larger one may become lukewarm, and the top container will probably hit room temperature or so. This is fine as long as you know it going in, as you have to play to its strengths.
The bottom container seems best suited for thicker, stewlike main courses - stew (of course), chili, butter chicken, curries, thick soups, chunky pasta sauces, etc. If it pairs with a side, all the better, as the large container above it can keep your rice or pasta or potatoes warm.
Above that, keep snacks that don't really need to be kept cold for the several hours they're likely to be in the container - vegetables, crackers, hard cheeses, and so on. Relegate cheese to the top container to avoid sweating.
My only real disappointment with the container is that it doesn't keep things piping hot by lunchtime - after a lot of experience with Zojirushi thermoses, which will keep undrinkably hot coffee too hot for half a day, the relatively weak heat seal on the middle container's lid is disappointing, as piping hot stew in the bottom container will be warm to hot in four hours and hot rice in the one above may just be warm. And that's inside on a desk, so if you keep it at colder temperatures outdoors I'd imagine it's thermal performance would be even more disappointing.