If you need to take out an opponent quickly, it can be very effective. When I used it, I usually changeed about half way through a battle just to get enough of an edge to turn the tide of the battle.
The damage part wears on you and the reputation loss can hurt if you're going to go shopping.
I used it occasionally with chaotic, neutral, and evil characters. I just thought, in a role-playing sense, that having a good character change into the Slayer would be out of character.
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.
"The only medicine for suffering, crime, and all the other woes of mankind, is wisdom." -- Thomas Huxley