{{accuracy}}<!-- according to the tooltips, a level 67 Felguard should have 1043 base mana, but the tooltip for Cleave says it costs 252 mana, which is closer to 24% of base mana -->
This is a great ability for solo and pvp, however it can cause havoc in groups using crowd control (CC) that breaks when the controlled creature is damaged. If you will be using CCs like Polymorph or Freezing Trap it is best to turn this ability off.
Similarly, if your tank has more than one creature attacking him or her, Cleave will often draw aggro from whichever creature the tank has the least threat on.