A few of your terms are a bit ambiguous here. The European Early Middle Ages runs to about the year 1000, by which time Iceland had only been settled a century, while in Scandinavia we count the Viking Age as extending to around 1050 and the early Scandinavian Middle Ages start there and extend to 1200; which is quite a difference since the year 1200 would be about contemporary with the earliest literature.

Then there's the matter of which literature; see, "saga" in English is a narrower term than it was (and is) in the Norse, where it just means 'story' or 'history' (or in the case of the Bishop's Sagas, a hagiography), so most of what the Icelanders wrote were "sagas" in some sense, while in English when one speaks of sagas it's primarily the epic stories and Kings' sagas that are meant; works like the Heimskringla and Íslendingabók. Another category here is the Íslendingasögur - Sagas of the Icelander's, which are about the early history of Iceland specifically. So "Icelandic sagas" could be taken to refer to those, or to any saga written down in Iceland, many of which describe events in mainland Scandinavia, or are translations of foreign works.

But in any case, since all of these have a connection to Iceland in their own ways, and treat events going back to the settlement of Iceland, they are all important in any case as they make up the a large amount of the available literature, not just on Iceland, but all of Scandinavia, up to the 13th century or so. But they have to be treated carefully and critically, as one does in history research. Ask who wrote this? Why did they write it? What are its sources? How long after the events it describes? etc. So for instance Sverres saga and several other sources on the Norwegian civil wars are contemporary with the events they describe and thus more trustworthy than (say) the stories about the founding of Iceland in Íslendingabók, written centuries after the events they describe. The stories of Ynglinga Saga, taking place even farther back and farther away are regarded as legend at best. Yet Yngvars saga víðförla for instance, despite being written in Iceland a century later about events in Sweden, and despite having fantastical and impossible elements in the story, still must be based on a kernel of truth since Ingvar's disastrous journey is the best-attested event of 11th century Sweden, with over 20 runestones commemorating people who died on it, carved by different people and spread over a large region.

So it varies and depends on what you want to find out about. (Particularly years/dates are very uncertain across the board) For example, there's no evidence any of the specifics in Njáls Saga actually happened. But the fact that Icelanders seem to have believed the story still tells us that the descriptions seemed within the realm of the plausible and that that sort of thing did happen. (similar arguments can be made for other sagas) However, the fact that Njál's is written down in the Christian era and the story is about pagans (or recent converts), one has to consider there could be exaggeration in how the people in the story are depicted as almost comically prideful and quick to violence.

Now for something completely different, there's the 12th century Niðrstigningar saga, an Icelandic translation/adaption of the Gospel of Nicodemus, a common bit of medieval apocrypha. But the translation introduces Satan as "iotunn helvitis hofðingi" - lord of hell's giants, surrounded by devils and trolls, and equates satan to the Midgard serpent, saying a lot about how Christianity was adapted into the Norse cultural framework in that era. Furthermore, Bullitta recently showed that the specific original text was a rarer 12th century French one - which underlines the connections 12th-13th century Iceland had with Paris, where the Notre Dame Cathedral School (becoming the Sorbonne in 1257) was by far the most common destination of Scandinavian students in those centuries (the diocese of Skara in Sweden even owned its own student dorms on present-day Rue Serpente), including Icelanders such as bishop Þorlákur Þórhallsson. Another example is the 13th century translation of Marie de France's Bisclavret into Tiódels saga and Darius Phrygius' writings on the Trojan war into Trójumanna saga. In the latter, the Judgement of Paris was changed from Hera, Athena and Aphrodite into Freyja, Sif and Gefjon. Which tells us that not only did educated Icelanders of that era have knowledge of Greek legends and mythology, but were seeking to equate their pagan gods with those of the Greeks and Romans, presumably to raise their status. So that tells us something about their knowledge and motives that may be worth taking into account when looking at other texts (not least the Prose Edda, which also mentions Troy and makes the same parallels).

So it is not just the context of Iceland at the time of writing that has to be taken into account, but the European context as well. Because by that point, neither Iceland nor Scandinavia were as isolated from the rest of the continent as people tend to think, and that would also influence how they viewed and presented themselves retroactively as well.

So in short though, very useful, but they cannot be taken at face value.