Any new deployment we are doing is going out on 6.0.x, the existing platforms are still on 5.6 for now - will look at upgrading within the next 6 months probably.
We use it and it works well for us. A lot of 200E and under, some 600D, and 1500D with about 20G aggregate traffic.
That being said, YMMV and please talk to your SE about its stability for the features you use.
Fortinet designed and built FortiOS
v6.0.1 to deliver the advanced protection and performance that standalone products simply cannot match. The services work together as a system to provide better visibility and mitigation of the latest network and application threats, stopping attacks before damage can occur.
Fortinet FortiGate-VMX v6.0.1
VMware vSphere v6.0/6.5/6.7
VMware NSX v6.3.x/6.4.0/6.4.1
NetX library version
For more information on the additional supported software, see the VMware Compatibility Guide.