Silicones smooth your hair by sinking into the porous parts, and helping to lock in moisture.  Dimethicone is the heaviest of the silicones used in conditioners, therefore having the greatest smoothing effect.  Because of their weight, they can be more difficult to rinse out.  I use a conditioner that contains dimethicone and really love it.  However, once a week (or when my hair starts to have that heavy build-up feel) I use a clarifying shampoo.  It seems to do the trick for me!

Hmmm I don't know much about this topic personally, but I will ask some of our Total Beauty editors (they can get access to experts) and see if we find anything out.