The show does seem to be following a rough historical outline. Ragnar, Lagertha, Rollo, King Horik and King Ecbert are all historical figures. I think that if you want to know how things are going to develop in the show you just need to consult google. Case in point? The first documented Viking raid was on the island of Lindisfarne, off the northwest coast of England. And that was the first western raid we saw in the series.
I've been watching since last season. And it's been interesting to see how things have developed. Some of the character actually seem to be developing some growth, which is refreshing. I had some skepticism about the History Channel producing an original program like Vikings. And the show really hasn't had a whole lot of star power attached to it. Most of the actors are relative unknowns except for Gabriel Byrne as Earl Haraldson in the first season, and Donal Logue as King Horik. Hell, I was pleased to see Vladimir Kulich in a couple of episodes of the first season since he played Bulwei in one of my favorite guilty pleasure movies, The 13th Warrior. And oddly enough, he played a Viking chieftain in that, too!
But Lagertha and Princess Aslaug? OMG. Both are just amazing.