I've been reading the Penguin Classics translation of Sagas of the Icelanders and I've been wondering a couple things about it. Clearly some things are definite fact (e.g. Harald Fairhair was definitely king of Norway during this time) and some things are clearly fantasy (e.g. the idea of shape-shifters and invincible berserkers), but are the stories of day to day life/politics accurate as far as we know? And a side question, is there any proof that the major characters (e.g. Egil Skallagrimsson) actually existed?
Edit: Title should be "generally regarded"
Before tackling your questions, it is important to understand that one major hurdle in the study of the Vikings is the lack of literary sources provided by the Vikings themselves. This is due, in part, to the Vikings use of a runic alphabet (the futhark) which was not suited for long narratives. The Vikings usually inscribed short statements, or past events, on stone, wood, bone, or metal. As a result, most of the sources used by historians to conceptualize Viking culture and society are based on sources from France and England (there are also a few sources from the East, namely, The Russian Primary Chronicle and the works of Ibn Fadlan), and both of these sources predominately cast Vikings in a very negative light.
So, while the Icelandic Sagas may not be completely factual, they provide another dimension to the study of the Viking Age. Moreover, the sagas often overlap with other primary documents on the Vikings, thus bolstering their historicity a bit. The Icelandic Sagas do have a few problems though. For example, Egil's Saga was most likely written by the Icelandic politician and poet Snorre Sturlson in the 13th century. Therefore, the Icelandic Sagas might be more of a reflection of 13th century Scandinavian culture, rather than a representation of 10th century Icelandic politics.
Ultimately, the Icelandic Sagas are an extremely useful primary source for historians studying the Viking Age. While they are not without their problems, they demonstrate so much about Scandinavian politics, society, and culture.
As for your final question, I am not sure if there is any evidence that Egil Skallagrimsson actually existed. I can't seem to find any mention of him outside of the Icelandic Sagas in my (regrettably few) sources. I would recommend reading more on Snorre Sturlson though, he has his own saga and it is quite an interesting read!
Sources:
Byock, Jesse L. Viking Age Iceland. London: Penguin Books, 2001.
Hall, R. A. The World of the Vikings. New York: Thames & Hudson, 2007.
McDonald, R. Andrew. The Viking Age: A Reader. Toronto: University of Toronto Press, 2010.