Well, it is unclear whether they get "drunk" or not but they do display hyperactivity after being exposed to intoxicating vapors of ethanol which is similar to that of what humans do after drinking too much. A recent study done by Karla R. Kaun et al shows that flies are attracted to ethanol just as much as humans are. Although humans have various reasons for ingesting alcohol, flies on the other hand, are attracted the rewarding effects that ethanol has on the brain. This attraction towards ethanol and the rewarding effects are so great; one could say that they are addiction to ethanol.
In the study done by Karla R. Kaun et al, they wanted to test whether or not flies displayed addition like behavior such as that in humans. They conditioned flies to be attracted ethanol by various means and tested them for addition like behavior by administering 100 V and 120 V shocks. They found that even after administering shocks to the flies, they were drawn to ethanol. Another test was done with the same voltages except ethanol was replaced with sucrose. This time, the flies only tolerated the 100 V shock and not the 120 V shock. This higher tolerance towards ethanol than sucrose could mean that they associate ethanol as giving them a more rewarding feeling than sucrose and that ethanol is worth the pain.
The flies were, as one could say, addicted to ethanol but why was this? Well, as we know, dopamine plays a role in the reward system and "ethanol amplifies the dopaminergic responses to natural reward and reward-related environmental cues" which causes this attraction and who can blame them, we all like to feel good (Karla R. Kaun et. al, pg. 3). Not only does dopamine play a role in the reward system, but so does memory of that good feeling. Karla R. Kaun et. al also found that the mushroom body and scabrous gene were required for the ethanol reward memory. By blocking various synaptic transmissions in the mushroom body, they found that the formation of ethanol reward memory "may be mediated by dopaminergic innervations of the αβ neurons (Karla R. Kaun et al, pg 5)." Karla R. Kaun et al also found that within the mushroom body, there was the scabrous gene that was required for the ethanol reward memory. It plays a role in this reward memory in that scabrous sends signals to Notch in which Notch mediates the reward memory. And so with the brain releasing chemicals that make you feel good and memories of that good feeling, who wouldn't be addicted to something that made you feel this way?
So why does studying flies and their addiction towards ethanol matter? Well, by studying flies and what influences them in their addictions, it could help researchers better understand human addiction and possibly allow researchers to find ways to help people with these addiction such as finding genes or circuits that makes a person more susceptible to being more addictive to various substances. By being able to identify these factors that influence a person's addiction, there will be better ways of treating a patient who has a drug abuse/addiction problem and better ways of treating the side effects of going off the drug such as withdrawal.