We are using ZoneAlarm Antivirus + Firewall for our employees who have BYOD devices. It is implemented at our one small branch office, where we do not want to purchase it because this is a temporary location.
This is a free version that lets us protect our users with CheckPoint security.
ZoneAlarm provides us with very good security and we are using both of the modules. This includes antivirus as well as firewall features, and it is very easy and stable for us.
We are doing regular scanning of our employees' BYOD devices and it's updated regularly.