Ring modulation, the 'black sheep' of the stompbox family, gets an airing here: Moog's MF Ring is based on the Moogerfooger MF-102 (allegedly the world's best-selling ring-mod pedal).
Juxtaposition of the freq and tone knobs yields a wide range of buzzy, chiming and metallic tones that can be tuned to be musical with a certain scale, but dissonant on notes outside it.
At the lowest mix-knob settings, you are getting subliminal ear candy - metallic resonance, for example - in the background behind your dry tone; but as you turn the mix up, you gradually lose that dry sound to a full-on aural assault.
What really appeals about the MF Ring is the fact you can use an expression pedal to control the freq parameter for sonic sweeps, or to switch between two keys.
If you want to add some (very) leftfield sounds to your rig, this is the way to go, but make sure you buy an expression pedal with it if you really want to freak out with the freq.