I have two 3601E Active-Active clusters and I upgraded from 6.0.5 to 6.0.7 over the weekend. In both of my clusters what was the master did not upgrade to 6.0.7 and I had to manually connect to the what was the master and force the upgrade. So other than the upgrade woes, I've had no issues with the software.
Upgraded our two locations to 6.0.7. No major problems, but a couple FortiAPs didn't fully come back (functioning, but FortiGate wasn't seeing them as connected). Power cycling the APs brought them back to connected status.