In response to Ogdahl's single card evalution of Haunter:

Haunter CAN do 50 damage for only 2 energy, BUT that is only if you can keep 
the defending Pokemon asleep.  His other attack CAN put the Defending Pokemon 
to sleep but it is an attack which means that you have to wait your next turn 
to use his Dream Eater.  That means there is a coin flip after your turn and 
a coin flip after your opponents.  A 25% chance of actually getting to do 
damage is not something worth risking.  I've killed many Haunter with 
Hitmonchans who he has resistance to just because they could not get the 
lucky coin flips.  I hope this helps you understand how sleep works.

-AntAu