The benefits of antidepressants relative to placebos appear to be correlated with the severity of depression (see <a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11799341&dopt=Abstract">http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11799341&dopt=Abstract</a>).
The evidence is complicated by the fact that subjects often know whether they are taking placebos (see <a href="http://depression.about.com/library/weekly/aa072202.htm">http://depression.about.com/library/weekly/aa072202.htm</a>).
Also, it should be noted that existing measures of depression are imperfect. Here's a study that shows an inverse correlation between suicide rates and use of Prozac/SSRIs:<a href="http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0030190">http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0030190</a>.
It appears to me that SSRIs (and maybe other antidepressants) were initially used in cases where they were clearly beneficial, and then the use expanded to a much larger set of cases where the evidence is ambiguous.
I have some experience with an SSRI. It took me 2 hours to decide that it was very different from a placebo, but it took me 2 months to decide that I didn't like the net effects. In particular, it caused reduced obsessiveness that was beneficial in a significant fraction of contexts (i.e. when sleeping), but not beneficial as a permanent effect.