I've been using a couple of generations of Zywalls. The one that this Fortigate replaced was a Zywall USG50 with a ton of features. It was extremely difficult to configure, and it had some flaky bugs. For example we could restore a config, and it would stop responding to just one of the static WAN IPs after about 3 hours. We'd restore the same config - no changes, and sometimes the issue would go away, sometimes not.
But the final straw was bandwidth. This remote site was supposed to be getting 60Mbps but the best we ever saw was 50. We chalked that up to the cable company. But then we upped the bandwidth to 110 and it was still 50. The USG 50 is rated at 100Mbps, but when we attached a laptop directly to the modem we saw 112. Horrid.
We hooked up the Fortigate and could only ping one of our 4 static WAN IPs. We tripled checked the config and couldn't figure it out, so we filed a ticket with Foritnet. The next morning we had a response, which we responded to. Within 30 minutes we had another response, which we also responded to. Then my phone rang; it was a Fortinet technical support engineer calling me directly to solve the problem. Yes, in 2018!
He diagnosed it 10 minutes; reboot the cable modem (thanks Shane). Brain cramp on my part; the cable modem ARP cache had the MAC addresses of the old firewall. Once we did that everything worked perfectly - we had done the config correctly.
Running a speed test there was absolutely no change in our speed readings than from connecting directly to the cable modem; 112Mps down and 12Mbps up.. This unit is rated for 950Mbps , so there is a lot of headroom.
With great performance and unheard of support, why wouldn't you go with products from this company?