Actually, it's not necessary, I just discovered the Hotkey toggle which makes it easy enough to disable the effect on the fly.
It were nice, though - unless I am missing something again - to have quicker access to opacity levels. Either by hotkey to bring up the slider or to step up/down directly. Or to have a command line parameter for setting up various presets. Different backdrops call for different opacity levels.
The SysTray is not readily available in many situations. Even if Win settings don't hide the bar, there are fullscreen programs that won't allow it to come up on hover. Just an observation from my user experience so far, but no big deal, either. Overall, I am very satisfied. Great tool, I only wish I'd come across it sooner.