So far as I know, they are a very nearly perfectly balanced source of both protines and carbs. They provide the most complete source of protine outside of meat products, because they are only missing one or two of the amino acids we need. If I'm not mistaken, they are as good, or better, a source of nutrients than soy. And they take on the flavour of other foods really really well.
You can use them as a side dish, as a thickener for soups, stews, and sauces, as a base or main ingredient in cold salads... the versatility of them is quite astounding. My personal favourite is to take a pan I've roasted some meat in, fill it with water to cook off some lentles, and use the resulting as a hamburger substitute in a 'meat loaf'. Every time I have done it it has turned out fantastic. I don't think I'll ever use hamburger to make meatloaf again.
Two of my personal favourte things about em: they are CHEAP! and they really really fill you up. I did a daily diet nutrutional analysis (at freetrainers.com) and found that if I could eat 11 cups of cooked lentils a day my diet would be 96% nutritionally and calorically complete. Unfourtunatly, I can only eat about one cup, tops, at a time before I'm full. The nutrition is there when I eat less of them, but the calorie count isn't high enough. A.K.A. its a good, healthy diet food.
I love lentils. This has made me hungry.... gonna go cook some up.