This one gets a lukewarm reaction from me. The idea is not a novel one (in fact, I'm pretty sure each Star Trek series has at least one episode about this). From "The Mannikin" to Dr. Jekyl and Mr. Hyde, it's been done. Not that this is bad in and of itself, but I did not feel this one put anything resembling an original spin on an old idea.
It follows the pre-constructed idea of "Good Self" vs. "Bad Self" to the letter. The "Good Self" is compassionate, but less vigorous than the "Bad self." The "Bad Self" is more emotional, less controllable than the "Good Self." Neither self can live without the other etc. etc.
The language was good I suppose, but I'm not a big fan of the "punchy" or "hardboiled" style.
Good reading by Ben, though.
ST:TOS The Enemy Within, Mirror,Mirror, What Are Little Girls Made Of?
ST:TNG Second Chances, Datalore
ST:VOY Equinox, Faces, Tuvix (in reverse this time)
ST:DS9 Defiant