Silicones are often vilified, but this seems to be based in rumor and myth. People have heard bad things said about them so often it just "sounds" true now. Silicones have tons of scientific research on them which shows them to be beneficial to the hair.

They might not be the best for people with very fine, limp hair, because this type of hair is easily weighed down, and many silicones can weigh down hair (though there are many that evaporate).

For those of us with dry, thick, hair that likes to explode into a dry mushroom cloud, this moisturizing weight is a good thing. It means our hair is surrounded in a very thin layer of emollient, helping keep it weighted. There are rumors that silicones "smother" the hair, but this can't happen because hair doesn't "breath". I have also heard rumors that silicones build up and make hair brittle. This is also not true. The ones that do stay behind after rinsing actually keep hair coated in moisture so it doesn't loose as much water.

For more information on silicones, you can check out Silicones in the hair products Ingredients Dictionary, or this entry from Beauty Brains: Are Silicones Bad for Long Hair?

I have been using products with silicones in my hair for the past ten years, and it is well past my waist, and shines all the way to the ends. However, I understand they aren't for everyone, and have included many combing and rinsingconditioners that don't have any silicones in them.