In my experience, players are not interested in weapons like these.
Anything with this serious of a drawback is immediately suspect, and when that drawback is possibly losing control of your character, which for all intents and purposes means not getting to really play the game, well, like Crothian said, who would want these?
If your goal is to use the promise of power to lure your characters into these dungeons. Why not simply tell them that deep inside the dungeon is a magical fountain that when they place their OWN weapon into it will imbue their blade with the power of a red dragon's soul?
My players would be much more likely to jump at that option than to knowingly quest for a weapon they may not be proficient in, and that might take them over. This way, it will be that much more of a surprise when the soul now bound in their weapon begins communicating with them.