It's fantasy to be sure. I stopped expecting historical realism after the 3rd episode. What I do feel it gets right is some of the ethos and adventuring spirit of the Vikings, as well as their disregard for Christians, at least in the early part of the era.
Aye for all me complaints it is cracking good stuff and it does capture the right spirit, in that way it's kind of like the first Conan film with Arnie. It's not Howard's exact vision and Arnie isn't his Celtic superman, but the spirit of the film is bang on. Mad Man Milius' finest hour.