I think he's a pretty good actor. I think he did a great job in The Departed and Blood Diamonds. I think he did pretty good in The Gangs of New York as well. It took a while to forget about Titanic, but I think he's done a pretty good job acting since then.
As far as playing a Viking, Hollywood has given us an incorrect impression of what Vikings looked like. I remember reading years ago that the average Viking was 5' 6" tall and probably somewhat skinny or malnurished. They didn't look like Hagar the Horrible with horned helmets. And while they were good fighters, many places just gave up the loot when the Vikings showed up to avoid a conflict with them.
As a person who has Norwegian/Swedish roots, I have read quite a bit about them. I remember getting into an arguement with a middle school teacher years ago about who discovered America. I pointed out that there was evidence of Viking settlement in Newfoundland around 1000 AD, centuries before Columbus set sail. Our history book made no mention of the Vikings in the area at all, and gave all the credit to Columbus. Since it wasn't in our history book, she said, I was wrong. I do remind people every Columbus Day that the Vikings were the first Europeans who truly discovered the New World.