What the others have said is true if you plan to grind mobs. Personally I don't like grinding mobs for hours. I only quest and run instances to level 60. With that said, I have found the pet tree useful and especially Soul Link, pet sacrifice and fel domination. Amazing for PVP which in my opinion is more useful than being the most efficient at killing mobs one by one. The pet sacrifice gives you unlimited mana when destroying the felhunter which is useful for instances.