We have been using this model for three years, to place a firewall between ISPs and our corporate network. As of now, we have configured some SSL VPNs on our end for our convenience.
Three years ago we encountered malicious attacks from the internet, most of which were Chinese attackers, so we deployed Cisco ASA to strengthen our network. Since the deployment, we haven't seen the risk we encountered before.
Manageability of Cisco ASA. It has a GUI interface, unlike the most of Cisco IOS. For beginners they can "sneak in" and apply the command and see the actual commands that the GUI launches. In addition, Cisco has the reputation regarding security.
There are more powerful firewalls, other than the Cisco NGFW, like Fortinet, Palo Alto and so on. I can't say Cisco is the leading firewall brand as of now, as the technology innovates.
No scalability issues yet.
I rate it an eight out of 10.
I am only handling or supporting the ASA 5520 model in our company.