I'm surprised you haven't received an answer to this yet! This isn't my area but I do have a well regarded textbook on hand (Bennett) so I'll use that to give you an answer.

The Vikings caused major upheaval in the aftermath of Lindisfarne in AD 793 until their assimilation into Christendom, both in overseas settlements and in Scandinavia itself, by around 1000 AD. Motivated by their belief in the important of reputation and legacy and their detailed knowledge of Europe (their trading partners) and its resources, a mixture of population pressure and political developments (kings flexing muscle, some adventurous individuals not like it), the Vikings began raiding overseas using their remarkably seaworthy vessels we now call longships. These vessels could sail the open sea, but could also raid inland in rivers since they sat so high in the water, and were small enough they could be beached and put back out to sea at any time. They were the perfect tool.

The Vikings tended to attack monasteries in particular, which may seem cowardly, but it was just smart really. Monasteries were defenseless and full of expensive items. Vikings were very good intelligence gatherers, and tended to attack targets they knew they could take. This is again, just intelligent practice. They also targeted realms with incompetent kings and towns riven by factional infighting. Their skill and knowledge as traders assisted them in this.

According the Bennett, the Vikings were actually very strong even against armed opposition: "They were sometimes turned away with bribes, but seldom by armed resistance." The places they targeted, even kingdoms with armies, rarely resisted them permanently. Wessex did, as did East Francia, but the Vikings were successful enough that they turned England on its head over the top of much armed opposition and carved out their own nation in Northern Europe, Normandy.

They took most of Scotland and Ireland, settled Iceland, Greenland and even temporarily settled North America (Newfoundland)! They overran Finland and fought their way into direct trade with Constantinople and Baghdad. The Swedes even attacked Constantiniple numerous times, and while they didn't take it they did extract various trade privileges, and apparently their physical size and fighting ability was so valued that the Byzantines incorporated many of them into the Varangian Guard, themselves Germanic and Scandinavian. The Swedes established a dynasty in Russia and captured Kiev in the 10th century.

The Vikings liked the places they took over and settled in, and assimilated pretty readily. In the east they converted to Eastern Orthodox Christianity, and in Europe they became Catholic Christians. Scandanavian kings continued to attack England into the eleventh century, but by this point the actual settlers from the previous centuries had mostly assimilated.

So yes, the reputation of the vikings as fierce warriors has a great deal of basis in reality. They traveled the world over open seas and attacked kingdoms and major cities, so they were obviously brave. They were apparently physically very strong and were very capable fighters, and their war record was impressive. Yes they lost against some strong kings, but I daresay home-ground advantage is always important in such things. It's not to fair to say they were glorified bandits: they shook Europe to its core in their time, they toppled kingdoms, established new nations, and enriched themselves greatly in the process.

Source:

Judith M. Bennett, Medieval Europe; A Short History (Eleventh Edition), 2011.