Very heavy duty glassware, but not so heavy to handle, lift and use.  The lids seal perfectly, and even with liquids, there is no leaking when turned upside down for a long period of time.  These containers also work very well in the freezer and move very easily to the microwave.  No staining when using red sauces, and they don't absorb scents or smells of the foods that you store in them.  The glassware and the lids clean very well in the dishwasher, and so far the seals are holding up very well.