As Memnoch said, one of the cons is that the form is unstable. While in slayer form you will notice that you take damage just by virtue of being in the form. I would suggest that if you are planning to stay in slayer form for most of a big battle that you stock him up with healing potions and ready your healers. The damage he sustains from this new ability can be pretty big. When i used this form I had the healers cast healing spells on him even if he was healthy at the begining of the round.
The pros- he can really kick some major ass. I used it in the mind flayer dungeon and he did almost all the work by himself.
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Balance