Is coffee really useful for alcohol hangovers When people experience a hangover due to too much alcohol intake the hours before, sometimes they are offered a good cup of coffee (at least in my country), not as the only or the main "treatment", but as a helping element in such a condition.

Is there any scientific evidence for this?
No.  Coffee is often offered to people perhaps about to pass out so that they will stay awake long enough that they can be kicked out of an establishment.  

Alcohol causes dehydration, electrolyte imbalance and taxes the liver.  To this extent standard medical treatment is fluid, electrolytes, and vitamins.  Caffeine is never used in a medical setting to treat alcohol overdose or withdrawal.