I really like Thorin. He's great, and his ability can be activated as early as second round. I almost always include him in any dwarf deck. Fili and Kili can activate his ability right away and so can cheap dwarves such as Dwarven Sellsword.
The one thing I don't like about him is his starting . 12 is very high for any deck, especially when one of the other heroes will most likely be Dain Ironfoot, which is 23 with only 2 heroes! To balance this out, I like to include Nori as the third hero to reduce your threat. With Dain Ironfoot, Thorin can have a 4! Slap a Cram or Unexpected Courage or him to also use that Dain-boosted 4! For me, Thorin is a all around good hero with an awesome stat-line and ability.