I want to use a MiFi with a wireless surveillance system, which can record 100s of GBs of video per month. If I send video to a PC all on the same network, does that count as data usage?
Traffic sent over the local network will not be counted against your MiFi/data plan. VZW charges for all traffic that is sent over thier wireless infrastructure/towers. If this your question is then that is the answer.
With that being said:
You can setup your cameras and disconnect the MiFi from the VZW network. There is a Disconnect button on the main page of the router admin page that allows you to do it. This would enable you to maintain the local network and confirm that the security system will work offline. After you confirm your camera to computer network is working you can reconnect it to VZW and see if you can access the computer / camera over the web.
As a local router the MiFi will perform just fine with your cameras and computers. There are certainly better routers you can purchase for that function but thats another subject. Its when you try to make your devices available over the internet where most people have problems. Since the MiFi is a 4G capabile device it is tied into the new SIM card and NAT network. In most cases public IP addresses are not assigned or accessible on the MiFi. This makes it very difficult to get through to your devices from a remote location (as I assume you want to do). Most users on this forum have not found a way around it other than reverting back to an older model like the MiFi 2200.