Of course the popular culture has always been strong for violent collapse and a Dark Age, but among historians over the past few decades the peaceful transition view has mainly held the field. Hogwash, I say. And so does English historian Bryan Ward-Perkins. The Fall of Rome and the End of Civilization (2005) is a tightly-argued little book making the case that the transition from Roman imperial rule to the Germanic kingdoms was profound and mostly unpleasant for those who lived through it. Ward-Perkins treats the documentary evidence, but his main focus is on archaeology. It is by comparing the material records of the Roman and post-Roman worlds that the dramatic impact of the empire's withdrawal becomes most clear.
I agree with Ward-Perkins that the evidence for a major decline in the quality of life for most Europeans is overwhelming. Other kinds of losses were equally profound; what percentage of the books in circulation in 400 CE survived into the Renaissance?
A much harder question is what happened to the European population. Yes, there are far fewer recorded archaeological sites in the post-Roman centuries, but that might be partly because early medieval people had so much less stuff that their sites are hard to find. Roman sites in Britain can always be seen on the surface, because of the masses of roofing tiles, pottery, and so on, but many Anglo-Saxon sites (even the royal hall of Yeavering) were only detected by aerial photography or geophysical prospecting. So maybe the people were there, just leaving lighter footprints. Yet there is other good evidence for a serious fall in population in the early middle ages. Much cultivated land was abandoned, from Wales to Tunisia. Cities shrank. Pollen studies suggest that forests expanded. If the population did shrink, as I think, it is also hard to say why. War generally does not lead to a fall in population across large areas; we can usually breed faster than we can kill each other. Pervasive disorder leading to malnutrition leading to low fertility was probably one cause, but there may also have been waves of epidemic disease, especially the bubonic plague of 542.
Of course, this does not mean that all the changes were bad, or bad for everyone. The collapse of the rather sterile classical artistic tradition -- a thousand years of satyrs chasing nymphs -- opened the way for the wonders of the insular style, and Romanesque architecture. Medieval society eventually became much more technically creative than the late Roman world, and inventions that the Romans had made but rarely chose to use (glass windows, water mills) spread widely. But not until the late thirteenth century would the mass of western Europeans have anything like the degree of civilized comfort enjoyed by their Roman ancestors. For other parts of civilization, the return took even longer. Not until the eighteenth century would European states have the power to undertake the sort of massive public works projects (roads, canals, harbors) routinely undertaken by the Romans, and the elite would not have houses as comfortable as Roman villas until the nineteenth.
The fall of Rome was one of the most dramatic events in the history of western Europe, and for most people one of the worst. If you want to know more, The Fall of Rome and the End of Civilization is a great place to start.