First and foremost I think the first three to four seasons of Vikings is brilliant tv viewing and contains much great storytelling on the part of its creators, with some superbly cast actors bringing the varied characters of Ragnar, Lagertha, Rollo, Athelstan, and many others vividly to life - my favourite is probably Gustaf Skarsgard's depiction of Floki, with more than a hint of Asperger's Syndrome (not sure if this was his choice, but he does it admirably!) - and also Linus Roache's portrayal of the scheming King Egbert of Wessex as a particularly self-interested, manipulative, and devious individual is very convincing.
The filming locations, cinematography and music are evocative. The scripting is on the whole very good and better than many other similar quasi historical or period dramas of this type. Scene setting including fights and battle scenes are well choreographed, and the extremely fine level of historical detail, which appears to be based on archeaology of the period and what is known about life in the time of the Vikings, make it all seem truly believable and very 'authentic'. Also commendable is how the Scandinavian mythology of gods and heroes, as opposed to the Christian beliefs then prevalent in Anglo-Saxon England, has been interwoven into the plotlines, without it becoming contrived. I became hooked after watching Season 1 and proceeded to watch the next three seasons back to back in the space of a week!
I have to say, however, after watching as far as the first episode of Season 4 Part 2, I have now decided to stop, for the simple reason that the level of acting of the new younger characters is not as strong as in the earlier seasons. This, allied to the fact that as a bit of a history 'geek' I was very aware as each season progressed, that actual historical events, known historical figures, and especially the chronology of those times, were being skewed more and more by the writer(s), a lot of it unnecessarily in my opinion, so much so that I no longer feel able to give it much further credence.
I was prepared to overlook a lot of this historical inaccuracy in the earlier seasons (e.g. King Aelle of Northumbria and King Egbert of Wessex were never contemporaries, Athelwulf's wife Judith was actually a Frankish princess and not from Northumbria, much of the genealogy of the Kingdom of Mercia has been altered to fit the storytelling, Ragnar and the historical Rollo could not in fact have been brothers, etc.) but the last straw was when it came to the supposed parentage of the future King Alfred the Great - I'm afraid that was really the deciding factor for me. I may start watching the later seasons at some point, but more for pure entertainment value and mild curiosity to see how this increasingly fictionalised account pans out, than as a believable historical drama.