I think he's by far the most interesting character in the game, and you can definitely see that if you do the Stormcloak line. The more you learn about him, the less you like generally, but I personally think he's awesome. Yes, he's a jerk and desires power over all, but he also has the potential to be a great leader, if he can learn from his mistakes. I think it's pretty clear that his victory and future as high king is going to be made canon, and I can't wait to see what Bethesda has in store for him, lore-wise. Very few chraacters, and especially leaders, in Bethesda lore are even mostly good. And the good leaders are never the most fun.
Anyone who says that Ulfric is just an evil racist maniac is missing a great amount of the story and isn't paying attention, in my opinion. And anyone who says he is a saint who only wants what's best for his people more than anything else are deluded. In my opinion, Ulfric cares for Ulfric first, the people second. The question is only whether or not that is acceptable to you. It is to me, and for that I will always be a Stormcloak on future characters.
You really don't learn much about him like why he can shout and why he'd use it in a fair fight.
If you speak to Ulfric during the main quest, he will tell you the whole story. When he was a boy he was summoned to High Hrothgar to become a Greybeard. He made the journey and spent most of his youth learning to shout and learning the way of the voice. However, when the Great War broke out, he did not find it right that his countrymen were giving their lives while he had to sit on the mountain staying out of it. Because of this, he left the Greybeards and renounced the way of the voice, deciding to use it to fight on behalf of the empire during the war. That is why he can shout. As for why he'd use it in a fair fight, I think that can be explained as well. Ulfric feels that in order to be seen as strong and powerful and capture the hearts of the people, he needs to create a good story surrounding himself. You see this a lot during the Stormcloak version of the civil war. His right-hand even makes fun of him for it. Ulfric probably could have killed Torygg in combat without the shout, but it was the perfect opportunity to strike fear into the hearts of his opponents and create his own little mythos. Who wouldn't want the guy who can shoot freaking lightning out of his mouth leading them? I'm not condoning what he did, because in the end it was NOT a fair fight if he used his shout, whatever his supporters say. But that's why he did it.
I do feel sorry for the Jarl of Whiterun but the fact is, he was given a clear choice. He could support Ulfric or side with the empire, and Ulfric would take Whiterun by force. He sided with the Empire. That is his choice and it is a valid one, but he knew what the consequences would be. So I don't feel TOO sorry for him. He put the people of Whiterun on the chopping block needlessly, out of pride. For all the talk of how evil Ulfric is, in almost every instance Ulfric has given his opponents the chance to leave or join with him. It may not be the greatest choice to have to make, but at least he doesn't run in and slaughter everyone unawares. The Empire sure didn't grant my character this courtesy at the beginning of the game.