"Euphoric, narcotic, pleasantly hallucinant" – that's what Mustapha says of soma. It's arguably the best tool the government has for controlling its population. It sedates, calms, and most importantly distracts a person from realizing that there's actually something very, very wrong – the citizens of the World State are enslaved. (Just think about the name; soma = "sleep" in Latin.)
John, of course, picks up on this in Chapter Fifteen; that's why he chucks the stuff out of the window in the name of freedom. This Mel-Gibson-in-The Patriot moment is not so effective, and mostly because of the way that soma enslaves its users – happiness. Everyone is trapped by happiness. And those are some tough chains to break.
Another thing to think about here is Mustapha's famous claim that soma is "Christianity without the tears." We get the "without the tears" bit, since a consequence-free high seems to speak for itself, but what does this drug have to do with religion? Well, as we've said, soma is an opiate that allows its users to be controlled. Brave New World seems to argue that Christianity functions in much the same way. It controls through pacification. It offers comfort, but at the expense of individuality. What do you think?