Spirit Pippin is one of the most powerful cards in the game. Period. If you don't use him in at least half of the decks you build, you're making a mistake, as he is incredibly powerful and versatile. Allow me to show you why.
First of all, his base statline is very powerful. While the 6 threat cost can be kind of high, you gain great stats in exchange for this. His 2 willpower is okay, but his high values of attack and defense are where his true strength lies. With his one attack, he can almost single-handedly destroy the average enemies in the game, and his one defense is also very useful for blocking the numerous foes that will come off of the encounter deck. In fact, he has such great stats that you will find yourself having a hard time figuring out what to do with him each round, so Fast Hitch is a good option for an attachment so you can get multiple actions out of him. However, his greatest strength lies not in his stats but in his ability.
His ability allows you to push an enemy back into the staging area once it engages you, while giving you the added bonus of increasing your threat by 3. This synergizes very well with his combat-focused statline, as having extra attack and defense when you will be fighting literally nothing is a great boon. Also, his stipulation that the ability only works with Hobbits allows him to fit into a wide variety of decks than he would normally not be able to. The threat raise of 3 also works quite well with his ability, as it allows you to use it more often. The more and more you use his ability, the more you end up using it. The ability to return an enemy to the staging area and let it contribute its is such a powerful one I expect it to receive an errata to limit the threat raise to one so that you cannot abuse it so easily by engaging more enemies and pushing them back, because your threat is rising over their high engagement costs. The added threat to the staging area from the enemies Pippin is pushing back can also lead to questing unsuccessfully, though, which raises the Pippin player's threat anyway! I suppose we will just have to accept that the card is brokenly powerful.
Final verdict: Ultra-powerful card that can go in any deck, and one that should get errata'd but won't.