Yes there are some pretty negative reviews in some of the speech and
hearing journals. Many studies have been done. Using headphones for a
long time will screw up your hearing... but mostly as you become older.
You don't notice what you miss, for the most part, but some people get
buzzing and other artifacts.
Most of the problem seems to come from high volume. People who use
headphones at normal volumes, and occasionally rather than every night,
seem to suffer litte.
The damage is minimal compared to the harm done in a car with a boom
box, or any other loud sounds.
Growing older is going to be a lot worse on your hearing.
An interesting study, is to have someone chart your hearing by frequency
now, then chart it annually or semiannually... you will see one set of
losses for normal listeners. Quite another set of charts comes from
people using headphones at high volumes, or from people who ride around
in cars or vans with those fantastic sound systems.