There are devils and demons, and weapons affect both. As for dragons, I am not sure if the bonus extends to big lizards as well as the true dragons, but I suspect it will or at least that it will affect the flying drakes. However, from experience I have found that these weapons don't do that much extra damage. If you get the weapon in a skill you've mastered, then go ajhead and try it, but don't have your main fighter use a mace, say, instead of his sword, just because the mace does double. (Say if you have 1 or 4 ranks in mace and 8 or more in sword).