Red lentils do not hold their shape as well as the Green or French lentils and go mushy if left simmering for too long. However, this is not necessarily a bad thing, as red lentils are fantastic for thickening up soups and sauces. If you want red lentils to keep a firmer shape, only cook them for about 10 minutes in boiling water.
Red lentils are the easiest to digest out of all lentils. They are a good source of protein too.