Most of what has already been said in review is true. The promotional cover is pure cartoon (the Vikings never had horned helmets nor were they leather freaks) and therefore quite deceptive, especially for those who get off on some serious blood and gore. As some are portrayed, Vikings never stumbled around with shaved heads unless they were naturally bald, and few would waste their time or lives tracking down an alleged omnipotent book regardless of its reputation because virtually no one could read in those days, and quite frankly it's likely few Vikings in that particular year in which the event takes place ever saw a book much less knew what one was. In fact the only thing in this film that has any connection to 'actual events' and 'true story' is the fact the Norse did reportedly land on Lindisfarne Island to forage and pillage whatever happened to be around which included a small Abbey.
The Vikings portrayed are not the Max Von Sydow-like tall blonds which is actually closer to the truth, looking rather gritty and grimy after a long nasty sea cruise which is also what a historian would expect to see back then. The frantic monks also look fairly realistic with appropriate ugly mugs and attire. In fact the only really pretty thing to look at is Elen Rhys who plays a wayward Celtic woman with an unexplainably unweathered face creeping around the countryside alone and captive.
However, negatives aside, I found the fictional story line reasonable and the setting appropriate with a plausible outcome if and only if one is ready for realism rather than romantic fantasy & adventure. The acting is good enough and reactive to the circumstances of having a bunch of violent fatalistic smelly dudes showing up uninvited, terrorizing the monks and other denizens into a hysterical frenzy. The ending, while creeping back toward a nearly implausible Hollywood feel-good conclusion, is rather refreshing after an entire session of woodsy dirt and grit.
Overall I actually liked it for what it was and aside from the aforementioned detractions give it four stars. But again a warning, this is not a pop-corning chewing, visually stimulating flick but rather a small introspect and speculation window of what may have happen in the late 8th century when the Norse began to rudely drop in on their across the pond neighbors in increasing numbers. That is, it shows what overwhelmingly most historians believe to be the beginning of the Norse invasions of Anglo-Saxon-Celtic British Isles. So while I give it higher marks for my tastes in this genre, I suspect many will not like the almost absence of glossy action-adventure, hence, this movie -- perhaps may feel their expectations 'burned'.