We also use Fortinet's FortiGate firewalls & would only be able to evaluate FSM if it supported Fortinet.
Earlier, it was asked what the appeal is. For us the initial appeal was that the FortiGate is a UTM with a wide range of sizes - suitable & cost-effective for very small offices and also larger offices. Now, after using them for 6+ years, the added appeal is that we know them, they just work, they are easy to maintain and the product has continued to evolve.
FSM supports Cisco, Checkpoint & Juniper. This thread is almost 17 months old. That's a bit of a red flag. I believe both Fortinet and Palo Alto have made significant progress in terms of firewall/utm market share since that time.