And, although the discussion seems to have moved a bit away from the "bliss drug" question, I wanted to comment on that, in light of this post, and add my own thoughts.
Let's imagine that someone managed to develop a machine, like Nozick's machine, which would be able to produce pleasure while we were plugged in. In case that we're unplugged from it periodically, the answer should seem clear: the pain about my miserable life where I haven't achieved anything, except hanging onto a machine, outweighs the pleasure by a large amount. Even if that pleasure is real- which seems very difficult to implement for me-, I would be so shocked about the waste of my life that the only possible answer for me would be to reject such a machine. Also, the pleasure would be non-natural (because induced by a machine) and non-necessary (by obvious reasons), and firmly rejected.
So, in order for the experiment to be more convincing, I shouldn't have the possibility to be unplugged. Once plugged in, plugged in forever. But then, which role does the machine take? It controls my world, it controls my life, my happiness and unhappiness, so it's God. When I become plugged in, the machine becomes my own, personal God; she controls every aspect of my experience.
In that case, the whole Epicurean argument- which states that there can't be any omnipotent God-, doesn't work. It's like applying Scepticism to a world in which in has (by whatever means) been proven that truth exists. Or Christianity to a world where it has been proven for certain that there can't be a God.
The Epicurean argument is alien and ineffective to a world where it has been proven that there is a God; but because this machine is God itself, we have to employ other philosophical traditions in order to determine if we should surrender to a God. Epicureanism is a real-life philosophy, not one built on hypothesis.
So tl;dr- such a "bliss machine" doesn't destroy the hedonistic argument, simply because the hedonistic argument never claimed to know the ultimate truth. It's designed to live everyday life, not to answer every possible philosophical question. And, as Don already stated, this mind experiment is- and will stay for the foreseeable future- a mind experiment. In case that someone discovers a perfect experience machine which provides only natural pleasures without any drawbacks, let me know- I would love to try it out