Pax Romana by Adrian Goldsworthy looks at this very question. Been a long while since I read it but I think his conclusion was that the pax Romana was a relatively peaceful and stable period compared to what had come before and after. That said, this does have to be measured against what was done to achieve it. Roman conquest could be incredibly brutal and they had no compunctions about wiping out entire people groups, enslaving and/or relocating them, and colonizing their own people. The period was also marked by a number of wars and rebellions but, comparatively speaking, these were pretty minor and very localized. Within a couple generations of conquest, most people had learned to accept Roman overlordship and focused their efforts on moving up in the hierarchy.