It depends on the food but I usually heat it up in the jar (without the metal lid of course). Dense thick stuff like refried beans can be tricky if you filled the jar to the top because it's hard to stir for even heating. For the most part just be conscious of the room you have to move stuff around and it's all good though. Obviously some stuff won't work well like pizza unless you wanted to roll it up like a taquito. I've done soups, rice, hot dogs, stir fry, corn, various pastas, curry, oatmeal, potatoes, you name it. Lots of things that don't microwave well normally are still just as okay in a jar such as steak bites, bread, fries, cream sauce, rangoons, or neatly stacked sliders

Oh damn butcher paper is a great idea! I'll have to grab some then I think that'll cover all my food related plastic bags uses