
Testing
The plates worked as advertised. The rims are ridged in such a way as to allow two plates of the same size to interlock together to form a storage container. The idea is to use the same plates you would use to serve food (or eat off of) to double as storage containers. Tina and I are big fans (or at least we use them a lot) of Ziploc brand food storage containers. They are low cost, convenient, and work great as lunch containers to take to work. But, a lot of the time, stuff doesn't fit conveniently in them, pizza slices, sausages, chicken marsala, etc. Plate-sized storage containers make storage of most meals fast and easy.
Microwaving food in the plates was also convenient. I simply popped the top plate off and repositioned it so the interlocking tabs sat on top of each other, opening up a bunch of vents to allow steam to escape. Reheating occurred quickly and evenly since the contents of the plate were spread out without having too much content stacked on top of each other (a common problem I have when reheating lunches that I pack for work).
The general feel of the plate reminded me of thick, heavy duty paper plates. The plastic has a matte finish that feels like you're holding paper, not plastic. It's also stiff and doesn't flex easily. Several other testers commented on how much the Hefty Serve 'n Store plates felt like a paper plate. One tester was so convinced the plates contained some paper that he didn't want "to take a chance with soaking the plate" for fear that it would rip or tear.
As expected, the plates hold liquid without any sign of soaking (they are plastic after all). We also hand washed the plates which cleaned up easily and nicely. I received some information from Pactiv when I called to ask about washing the plates. Since the plates are designed to be disposable, there will most likely be some reduced performance in the interlock mechanism of the plates (i.e. they won't fit together as well) with repeated washing. Washing in a dishwashing machine is not recommended due to the high temperatures that a dishwasher reaches.
I should also mention that the seal made by the interlocking plates is not water tight, but works pretty well to prevent liquid from escaping if it sloshes around a bit. Just don't fill it with soup and then tilt it.
Dinner
I brought over a stack of plates to a friend's place, and we grilled up a couple dozen burgers and sausages. Service was handled by the plates and they worked flawlessly. Most of the testers ate standing up holding their plate with one hand and accessing their food with the other. None of the plates bent or flexed under weight and the plates were deep enough to hold corn on the cob without the eater worrying about the corn rolling off the plate. The rims of the plates were both stiff and comfortable making the plates easy to hold.
We then served up Korean BBQ short ribs and some of the testers used a metal knife to cut the short ribs. The plate held up to the cutting just fine. It seemed that the plates worked at least as well as other heavy duty plates from such companies as Chinet or Dixie.
At this point, we tested the crux of the Hefty Serve 'n Store usage model: storage of leftovers. We took combined the leftovers from the plates we used for serving onto a couple plates and used the newly emptied plates to cover. At first we tried to shove too much food onto one plate, but the lid wouldn't close. After distributing a reasonable quantity of food onto the plate, the other lid snapped into place without a problem. After enclosing all the leftovers in the interlocking plates, they stacked on top of each other and went into a bag for easy transport back home to my refrigerator.

When we first heard about the plates, we thought, "Why can't we just use plastic wrap to seal the plates? Is it really necessary to use another plate to cover your leftovers?" Well, usually plastic wrap doesn't stick or seal well when used on paper or plastic disposable plates, so you have to wrap all the way around so the plastic will stick to itself. Also, once the plate has been wrapped, stacking plates doesn't work too well when the only thing separating the bottom of your top plate from the bottom plate's content is a thin sheet of plastic. The interlocking plates solved the problem for us and we efficiently stacked the leftovers without worrying about tilting plates or ill-fitting plastic wrap.
Conclusions
After using the Hefty Serve 'n Store tableware for a couple weeks, we are convinced that the product lives up to its advertised promises - plus they're quite convenient and the plates are high quality for the price ($2 for a 24 pack of 9-in. plates). Also, the bowls interlock with the 9-in. plates to form deep dish storage containers. The plastic is solid and has a nice textured feel (as opposed to other brands with thin flimsy plastic that might not hold up to a hearty plate of food). These plates are an obvious choice to serve food from when you've got an informal party.

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Update 2008-05-27 - The Hefty Serve 'n Store product line seems to be discontinued.