The real magic in an optical compressor as an audio device lies in the photocell.  The photocell has a gradual and predictable release curve, which is very gentle and musical.  The first 50% of its release occurs very rapidly (generally measured in tens of milliseconds), and a complete release that can take several seconds.  This type of release curve is generally not found it typical solid state or tube designs.  In addition, the cell exhibits a phenomenon known as memory. The release time of the cell is dependent on the duration and intensity of the light to which it has been exposed.  What this means is that when the input signal has been quite loud, and the compressor has been working harder, the release time will be longer. This interactive release automatically smoothes out the kind of uneven volume fluctuations that can result from other forms of compressors.  In the end, however, the differences must be experienced to be fully appreciated.

Also worth noting that some inexpensive compressors on the market now claim to have an optical emulation mode available.  In many instances this is just a slower attack and/or decay time and really doesn’t properly emulate the behavior of these optical devices.