This question is, I think, one of the greatest reasons to study the Viking Age. Because it forces us to ask, What was the Viking Age really about? Your question puts immigration and settlement at the forefront, but it also raises questions about violence, about entrepreneurship and technological advance. It really strikes at the heart of the historical endeavor: How do we measure progress?

The yardstick that you mention—things that lived on into modern times—doesn't seem to be the best possible measuring tool to me. In part that's because it assumes that the present day is necessarily better than the past, and in part that's because it assumes that only the things that survive into the modern period are good and worthy of study. And, in fact, anything that's survived since the Viking Age actually depends just as much on the decisions of people who lived during the thousand years intervening between us and the Viking Age, as much as it depends on the actions or achievements of the vikings themselves.

But I study the Viking Age because I do think it has a lot of intrinsic value for researchers and even the public today. There was a lot of violence then, just as there is a lot of violence now, but what did people living in the Viking Age get out of it? Well, among other things, they got a class of warrior elites who gradually consolidated power and started calling themselves kings.

And to fit in with the kings that ruled over their neighbors in Western Europe, these new Norse kings adopted the religion, the literary culture, and the diplomatic language of Latin Europe. This wasn't merely about assimilation or mimicry, because they adopted all these things on their own terms and transformed them as a result. The sagas and the whole corpus of Norse mythology, for example, would have been lost forever without a few churchmen who adapted the literary culture Latin Christianity to serve a completely new set of goals. Or from a more European perspective, the Investiture Controversy, which was an early statement of the ideal separation between church and state, was just as much about defining a right theoretical relationship between emperor and pope as it was about negotiating a practical focus on Christianity at home or on carrying Christianity abroad into the viking heartlands of Scandinavia.

And at the same time, to maintain their power at home, the new social elites adopted or developed new systems of political power and economics. The kingdoms that became later cores for the Scandinavian nation-states were all developed during this period. The new elites' power rested not only on their evolving relationship with the church, but also on their ability to foster relationships with merchants and craftsmen, with the people who made up the early cities of medieval Europe. The burhs of Alfred the Great were, after all, but pale shadows of the bustling urban centers of Norse Dublin and York.

These things, of course, relied not just on the top-down prerogatives of kings, but also on the diversity and entrepreneurship of the people who lived under them. This is, I think, the greatest legacy of the Viking Age. It's easy to think of the Viking Age as a thing, as a period when some sort of primal Scandinavian culture was let loose upon the world. But it was really a period of enterprise and adventure, and much of the world as we understand it—a world of cities, states, religion, and education—was defined during this period in ways that have ended up to be enduringly meaningful, even if nobody during the Viking Age itself could have predicted the result.

That's not terribly specific to the UK, because I think the legacy of the Viking Age is much larger than just the UK. The Viking Age was about new communities that were more maritime- than land-based, interregional rather than proto-national, about trade and exchange rather than surplus-hoarding and production, about developing new ideas regarding who rulers were and what the role of religion should be in a society and state.

The results weren't always positive, and many people suffered terribly as a result of the Viking Age and the violence it entailed. But as the modern inheritors of the Viking Age, we now have so much to draw on—a period when experiment, accommodation, and innovation were much more important as than conservation of traditional power bases and the exclusion of outsider groups. This is, I hope, the legacy of the Viking Age that we will choose to foster today. And it's the reason why I think that what I do as a historian is something more than a thought experiment but is in fact a potentially world-changing endeavor.

Some very influential cities have been founded by the vikings raiders, depending on what you define as 'good'- yes good has come out of it. A lot of these settlements were trading centers that gradually expanded, the citizens stayed there and they became the towns we know today. The Vikings founded many modern cities in the UK Yorkshire, Nottingham, Derby and the most important one in my opinion; Dublin.

While people lived around the Dublin area pre vikings, Dublin the settlement was founded around the 840s by viking raiders, texts describing this time such as the Annals of Ulster refers to the vikings as "heathens" - still loyal to the pagan religion when they settled there. Dublin became at the time an important center for trade (mostly slaves though - some taken to settle Iceland). It also had an impact on the sagas, the famous saga kings Olaf the White and Ivar the Boneless are said to have ruled there at some point. Dublin flourished in trade and later evolved to become a very important center for learning in the medieval period. It was obviously a very important (but moderately sized) city at the time it was founded - today it is the capital of Ireland and a very large and important city. So in that sense yes, the humble trading center lived on to modern times to become something very positive.