I agree with the other posts. The prescribed upgrade path and the release notes are very helpful. I have upgraded quite a few firewalls from 5.2 to 5.4 with no issues. I would always backup the configuration before and then again after a firmware upgrade.
Concerning 5.6, many folks wait for a good while before upgrading when new new major update comes out. The thinking is that it is best to avoid the bleeding edge of technology. This is especially true when it applies to enterprise environments affecting a significant number of users.
Theoretically when a new firmware version like 5.6 comes out it has a lot of new technology without a lot of real-world experience. While Fortinet obviously tests their new firmware extensively, it is hard to test for every combination of settings and network usages. As each incremental update is released, i.e. 5.6.1, 5.6.2, etc., more and more issues are addressed and the firmware solution becomes a more proven and a more reliable solution. I suspect that the average newer / supported FortiGate's are running on the 5.2 or 5.4 firmware at this time.