After the release of the v5 updater I decided to give FM12 a go on the Gold Master of OS X Mavericks.
One issue that I've seen is that the styles applied to fields don't always appear to stick. These are fields NOT using a FM12 layout theme (using Classic) and have applied my own states to the fields. A few times I have seen the styles vanish and I have to re-apply them. The different states (hover, focus, pressed, normal) seem to have changed a bit as well.
In browse mode, clicking on white space made a couple of the fields "pressed" state trigger. When the mouse was released the affected fields did not re-draw properly. The top borders were clipped and would not restore to proper state without dropping into Layout mode and back to Browse
Trying an Undo command also jumped back 3 actions rather than just 1. I wasn't completely paying attention when I hit the keystroke so I can't completely confirm that one but multiple things moved when I was expecting 1 field to shift back.
Still a work in progress. Will wait a bit before installing on the primary drives.