AIM for safer skies!
A: Many people have made the switch from other systems to QGIS, however it all depends on your specific workflow and what you would want to achieve. Actually many people even suggest somethings are much easier to do in QGIS.
However, if you have investment in ARC datasets and specialist software packages you should consider all the costs of switching. Try and make a cost benefit analysis of both offerings to find the TRUE TCO (total cost of ownership) before making the comparison.
There is an adage in the software industry: “there is no such thing as free software…” The staff costs, time spent, migration costs etc all stack up and need to be accounted for.
I just want to add that in many cases you don't even have to get rid of your software that you already own, let's say you own ESRI software but don't have extensions available since you didn't buy those. You can compliment that by using QGIS.
My mantra is always to use the best tool for the job, it doesn't matter if its free or not. Of course you may have restrictions in terms of what you can use and then you will need to work with what you have available and at your disposal.