These Forest Spirits make hard hitting close combat units. Their greatest attribute is their toughness (T5 is fairly rare for units). They are well protected with their armour save of 4+ and ward save of 5+ but, given the prevalence of Great Weapons and the Tree Kin's low initiative, they must be used carefully because they are a big points cost investment and you will probably only want to field a single unit (due to points cost and purchase cost !)
If you do field a unit and your game is 2000pts+ I recommend you take 7 Treekin. It sounds excessive and expensive but will make them more able to plough through the enemy quickly. You do not want to allow you foe any more time to take your Treekin down than you have to.
With 7 Treekin you will have a unit strength of 21 which, after a round of combat and some kills, should be enough to rout the enemy unit in turn 2 if not turn 1 (through Fear).
Remember, even if the Treekin take wounds, they will need to take 3 before a model is removed so the unit will benefit from it's unit strength and more attacks for longer than single wound troops.
Note also, that TreeKin are immune to psychology, so no panic, fear, terror etc.
Treekin seem expensive but if you're asking "should I use Treekin" then the answer (from me) has to be yes.