With a fixed Public IP address, you can connect to your device from any machine anywhere. You do not need to set up a VPN tunnel. This is an ideal solution for customers who do not have a firewall or do not require to set up a VPN tunnel of some type. However, as it is a public IP address, the device is open for anyone to connect to if they know the IP address.
Therefore, security is down to the customer. If you have a 3G/GPRS router then it is highly recommended you set up a logon page so that unauthorised access can be stopped.