CONTAINS SOME SPOILERS
Good but not great. As a fan of the original series, and anything Viking related, I came away feeling the movie should have been an hour longer in order to flesh everything out. As others have said, it just feels a bit rushed, and I was often finding myself asking, "wait, what just happened, and where did that come from?"
At first I was a little annoyed that Aethelstan turned out gay, because I was afraid Netflix just added it in. But it actually turns out to be done in a way that actually matters in the movie. Further, having not read the books, from what I can tell he was gay in the books as well. So bravo to Netflix (wait, what did I just say?) for at least sticking to the source material. It did seem out of place a bit though, since there was never any hint of him being gay in the original show.
I started enjoying the film more as it went along, as it became a bit easier to follow. But it still felt rushed, and there was little to no character development, which isn't much of a issue if you've seen the original series.
The film also lacked the strong female characters (Brida, Stiorra, Skade, etc) from the original series, and it seemed somewhat out of place that Uhtred didn't sleep with anyone, marry them, and then get them killed, although there is a mention of the demise the women in his life seem to have.
I enjoyed the final battle, and the ending itself, even if the battle could have used still a bit more strategy (it was there, just lacking in my opinion). And while others have scoffed at the final scene, I rather enjoyed it and would certainly like to visit. All in all, certainly worth watching - maybe twice since it goes so fast - but just not quite what it could have been.