The show is great. A few seasons in it basically changes which main character(s) it mainly follows. A lot of people didn't like that, personally I still found it enjoyable (but not as good as before).
And while it does include some historical characters and events it's mostly fiction.
I really liked the beginning of the show, but stopped keeping up with it about halfway through season 2. I get that the mysticisim is something that was real to the vikings but it started to weigh into the plot a bit too much for me to keep enjoying.