Oh I'm sure it's not particularly accurate in every respect, but then, it doesn't pretend to be. From what I've gathered, though, there is in fact a fairly high degree of accuracy in terms of costuming and props. They do have historians constantly on site, I believe. I'm sure plenty of stuff is embellished (Show Uppsala does not seem to be as flat as real life Uppsala ), but I really don't care. I love the show for its characters, both in writing as in acting, their development within the storyline, their dialogue, etc. To me, it's sort of like the way I view the LotR films; they leave out or change a lot of details that I as a book nerd find important, but they do manage to capture the right atmosphere and themes. Vikings does what I feel like a show like Game of Thrones tries to do: it contains characters that actually feel rounded, with interesting motivations and twists. The character Floki, to me, is easily one of the most interesting TV characters I've ever seen. Ragnar, too.
There's things that I feel they do so well, like representing the Viking oral traditions (giggity) during night-time tale-telling, the idea that the Vikings gods are alive and constantly walking among mortals (there's a wonderfully mysterious representation of Odin in season 3), the humanization of Viking culture (they're far from the savage, pagan raiders they are traditionally presented as), the portrayal of contact between pagan and Christian, the way they influence each other, similarities, differences, etc.
Timelines don't always add up (King Ecbert was not crowned king until two years after he shows up as king in the show, for one), but changes made serve an interesting narrative.
I love the show. Go watch it.