Most of the time its fine to microwave in them. Depends on what you mean by plastic containers...? Like the one that it comes in from the store? Take it out. The ones that you put your leftovers that come with a lid? Take the lid off and microwave it.
I wouldn't heat up any liquids in plastic but it just depends on what you have around you and how hungry you are and if you want to re-use it if it's not hard plastic I wouldn't risk it. If it's flimsy Tupperware plastic it will probably burn and warp it.
