The Scythian said:
I started as the petty king of Jutland. Wiping out the Knytlings in one generation was sweet.
Oh, I wanna do that! I usually play Sweden or some vassal, and the damn Danish insist on being a huge boulder in my path to the continent. I will have my revenge ...
In The Old Gods, I came pretty far with Svithjod. You start out needing just one more territory and a wad of cash to found the Kingdom of Sweden, so within ten-ish years of raiding and subjugating I was the first King in the North.
That title made things absurdly easy for a while. I could suddenly vassalize every count in the de jure Kingdom. Then, once I hade those 8+ territories under my banner, most of Norway joined as well because they feared our power! Wham, within three weeks of founding the Kingdom I ruled most of the Scandinavian peninsula, with no blood spilled! (Hardly the Viking way, but meh

)
The next fun consequence of that was that I suddenly controlled two Holy Sites of the Norse faith, so once I made up a reason to invade Sjaelland and take a third, I could reform the faith. I was suddenly the head of the religion, like a Viking Caliph, and could wage Holy Wars on pretty much all of Europe!
That was all done in the lifetime of the original starting character. His prestige and piety at the time of death was pretty silly!
TL;DR: Starting out as Svithjod can give you a big boost at the start. Avoid it if you don't enjoy easy mode ...
BTW, have any of you figured out how to get rich off looting? After the first few years I kept getting stomped on by doom stacks of 12-15 thousand troops wherever I went in Europe =(