We have to preface this answer by stating we are not lawyers or legal experts; so do double-check the rules, laws and regulations wherever you live!
Generally speaking, no it is not legal to use your laptop when you are driving. If the cops see you doing so, expect a ticket for distracted driving at the very least. You could also get a ticket for obstructed driving if the police deem the laptop to be blocking your vision.
Laptop mounts are much better used when you are pulled over and are no longer driving. For example, if your business sees you making house calls, maybe work on your laptop before puling away and driving to your next stop.
Just use common sense and don’t distract yourself whilst driving and you should be cool.