Here's my opinion on the subject.
If you just love the work, and can't say enough good about it, that's a
good case for public. It helps the rest of us who don't have time to
read everything pick out the good stuff.
Really detailed analysis should probably be private... most people on
this list don't have time to read detailed analysis of someone else's
work.
Harsh criticism should DEFINITELY be private. I've seen some scathing
critiques of people's work posted on the ML, and that doesn't help
much. First of all, the author gets offended that you shot him/her down
in public. Secondly, because they are offended they usually winds up as
a flame war, and any good points you might have made are lost along the
way. Everybody loses in that case... author, critic, and the rest of
the ML.
A little constructive criticism on the ML is good, but I suggest that
any critique posted to the ML should be positive in its overall tone.
Opinions?