The centralized management platform works fairly well, and the fact that licensing is simple (as in, UTM and VPNs of all sorts come with the default licensing) is nice. Also the price is a good selling point - it is a lot easier to get a small client to pay 200 bucks for a firewall than it is to get them to pay for a cisco ASA, for example.
Configuration wise, it is fairly easy to configure VPNs, routing, new interfaces (and vlans), and add new rules through the GUI. The GUI is fairly lightweight and pretty intuitive. You can also do a lot of troubleshooting through the GUI.
When you turn on all of the UTM options, the firewall slows down and processes traffic much slower. Also, there have been a lot of issues with the url filtering service in my experience. Also, fortigates do not like nested group objects, they will cause all kinds of configuration errors on some versions.
However, my biggest complaint would be the CLI. The policy configuration is pretty clunky as it is in xml style format. You have to type every line one by one, and there isn't a very quick way to add a large number of new objects. For example, to add one object it looks something like this:
configure firewall address object
set subnet 192.168.1.0 255.255.255.0
set associated-interface DMZ
The first and last lines don't need to be repeated, but the other 4 would need to be repeated for each object.
For a firewall, it is OK. Not great, but not terrible. I could name half a dozen worse firewalls off the top of my head. Clients like them for the price.