I have seen really amazing and really crappy GoPro footage. The super high def 2K and above advertising clips that GoPro runs are filmed on the highest resolution settings and professional post produced. I have never seen an amateur get that level of result in color and clarity.
That said, you can get some results without a post-production crew, OR you can get some crappy stuff too. Where and how you mount the camera is key, as well as the settings you use. Many people film to document in case of an accident, so production values for that use are not a high priority. My motocycling buddies who are not very tech savvy tend to produce stuff that is very flat (color-wise), low res and jumpy. Another friend of mine just made a killer movie of his houseboating trip to Lake Powell. A third techy friend makes films from his drone that are cinematic and jaw-dropping.
I am finally on the cusp of buying a GoPro myself after seeing some decent amateur footage.