The Treebeard mini promo expansion isn't really a required expansion. Treebeard is a huge part of the Lord of the Rings, but in the game their movement is so limited it is really hard to get them into the game in a useful manner. They have to link up and putting more on the map is really up to the opposing player (which they normally won't do).
Treebeard is a leader but can also add to the combat strength (number of dice rolled), which is something most leaders do not do. His limited movement hurts his usefulness. The ents in general are a sometimes helper.
I would say get this if you need to get everything. Otherwise, pass this right on by.
The components are a character card and miniture Treebeard. I have the painted version, but from my understanding the promo comes unpainted. The character card matches the rest of the game. The minis are top quality.
N/A. The rules are printed on his card. See flow of the game.
Flow of the Game:
If Gandalf the White is in play and a Companion is in Fangorn, you may use a Muster Action Die result to play Treebeard in Fangorn.
Treebeard is a character and acts as a leader. He can't really move so his attack is pretty limited. If he is in Fangorn/Rohan, he adds one to the combat strength (Leaders don't normally do this).
Should I buy this game?:
No, it is so hard to get Treebeard into the game and his function is pretty minimal. I can't forsee any reason to purchase this item unless you just want to complete everything for the game.