I'm afraid you're going to have to be a bit more specific: is there anything in particular about the Holy Roman Empire? By 'work' do you mean how was it administered, the election of the Emperor, why member 'states' like Bavaria and Saxony didn't just leave, or something else? How the Holy Roman Empire 'worked' can cover a pretty broad range of topics, and I don't want to prattle on for pages about something like administrative and court structure, if you're not interested in those topics.

However, to put it in general terms: the Holy Roman Empire worked because it offered more to the polities that were part of it than it took away. You'll notice that bodies which--while technically part of the Empire--but gained little tended to drift away (e.g. the Netherlands, Switzerland), but most of the German lands remained within the jurisdiction of the Empire. To the German princes, being a part of the Empire meant security from foreign invaders--as the Emperor was expected to help defend his subjects and system of Imperial courts provided ways to settle grievances and harms outside of war; greater political power, since each estate had a voice in the Imperial Diet; and greater prestige.

To me, Voltaire's quote is one of definitions more so than an indictment of the Empire itself. What does it mean to be an empire? Or holy? Or roman? The Holy Roman Empire was--indeed--ruled by an Emperor, but when we say it 'was not an Empire' do we mean that the Emperor wasn't an absolutist monarch like the French kings? When we say it wasn't holy do we mean that it was secular, or is it mean more to argue that a 'holy' state should be a theocracy? When we say it wasn't roman, do we simply mean that the Holy Roman Empire did not rule the city of Rome? All of these accusations are questions of defition, and--from a certain point of view--the Holy Roman Empire could well be considered, holy, roman, and an empire.