I'm presently in a mode where I carry in a lot of leftovers. Sometimes I'm in salami-nuts-cheese-chocolate mode, or a Greek yogurt mode, but not the last year or two, for whatever reason.

I don't carry any outright liquid unless it's in a screwtop container. Anything with a liquid sauce gets tossed in with rice or pasta that will gum it up enough it doesn't leak.

Today I have penne and meatball marinara in a Sistema soup mug with a handle, that I got from Safeway. The lid has three latches and a steam vent. It's about 2/3L. It has three latches on the lid, and the lid has a gasket. I have a second one of these. These don't nest at all, which is kind of annoying, but they work great otherwise.
http://sistemaplastics.com/products/...edium-soup-mug

We also have some smaller 1/2L versions with no handle or vent that are presently holding formula powder in the diaper bags but will be returned to lunch service one day. The smaller ones also have a separator tray and a little spoon.
http://sistemaplastics.com/products/...reakfast-to-go

With plastic containers, oil on the surface of your food - like in Parmesan cheese - can exceed the melting point in the microwave and leave a permanent bathtub ring. Plastics also pick up stains from food, if that bothers you. We are presently annoyed that our baby bottles have picked up an orange tinge in the dishwasher for that reason. Neither of these things are really a problem unless you're picky.

We also have a set of the Rubbermaid plastic ones with the flexible lid. Not a giant fan, they don't seal too great, don't stack very small, and they crack up after a couple of years through the dishwasher. They're not a huge improvement over the Ziploc tubs. I don't think we will be buying more.