Beorn is both one of my favourite heroes, and one I feel is sorely under-rated. I picked him out for both those qualities in the wrap-up post for the Against the Shadow cycle in The Line Unbroken.
So let's talk a bit about Beorn. Now, to be sure, there are other ways you can get 5 on a hero these days, and those heroes can then be boosted further with other attachments that Beorn can never have. And then there are other effective Sentinel defenders, with more and who can actually be healed when they take damage. But how many heroes can have 5 right at the start of the game? Gimli, Lore Faramir and Éomer can do it, but those are all conditional, and I can't think of any others. Beorn is always 5 with no conditions to fulfil. Even if you ignore his ability to defend without exhausting, you could simply use him for that 5 and 10 as an archery sink. but then if you do use his ability to defend without exhausting? Sure, there's a time limit on how long he can do it, but the early-game is the hardest part of this game. Let your initially weaker heroes and allies hide behind the bear as he tanks absolutely everything for the first couple of rounds and kills things with his giant claws, then let other characters take over defensive duties and Beorn can continue to tear through enemies. Start solid and you're likely to continue that way.
All of the above considers Beorn purely in a vacuum, but this need not be the case. If you build around him you can mitigate some of the problems. Landroval can be used as a reset button, as obviously can Fortune or Fate. Expensive to be sure, but they bring back the bear with all or almost all of his again to continue tanking literally everything on the board. Add in the ability to redirect damage to Dori (who unlike Beorn, can be healed), simply cancel the damage with Close Call or Honour Guard, or reduce enemy attack with effects like Horn's Cry and you can keep your friendly neighbourhood skin-changer alive and well and covering all your defensive needs potentially all game long. And don't forget that things like Sheltered Valley or Make Camp can heal Beorn since they're encounter card effects, not player card effects! Not to mention the occasional cry in my games of "Oh, everyone's text boxes are blanked. QUICK! HEAL THE BEAR!"
Overall, merely as a massive early-game boost Beorn is a beast, and when you actually include cards which support him he can get truly monstrous, not to mention the fun factor. I also consider Beorn the #1 hero in the game for pure unadulterated fun factor. A couple of decks using this hero: