I believe that missiles are extremely versitile and that some of the other weapons are extremely over priced. (35 points for a las cannon?! Also, why do space wolf long fangs get them for 25? Classic example of codex creep.)
My first devastator squad is therefore made up of most of the missile launcher troops which I had, including a few very old figures. The unit is not fielded at full strength, as the other squad members have already sacrificed themselves in battle, taking shots intended for the heavy weapon troopers.