It was both. On one side, it was not any less important for Western history than the foundation of the USA. Once the revolution had taken place, there was no going back to the status quo of before again, despite all attempts for restauration. Without the French Revolution, there would have been no liberalization and democratization of Europe, even if that took a while.
On the other side, it can serve as a warning: When revolutionary ideas are taken too far, and when old structures are clashed too rigorously instead of being reformed, even a revolution in the name of freedom can become a tyranny and a horrible massacre.
I think the revolution teaches us some lessons, one of which is that the state is not supreme nor infallible as some may believe it is. I however also agree that the French revolution was very bloody and the revolutionists themselves have been consumed by greed and power as executions happened here and there, seeming to forget that they were trying to fight against those same notions in the first place.
It was a political revolution, not an intellectual one (the intellectual one called the Enlightenment had already started as early as Louis XIV's reign).
I think the prime cause for the revolution was the backwardness of the French political system. In many ways it was still medieval: all the important ranks and privilidges were held by the noblemen, even though they held lost both their source of income and their military power (the king now had a standing army). The bourgeoisie on the other hand had accumulated much wealth and a high status in society, yet they had no significant influence in the country's government or the bureaucracy.
It was just a matter of time before the revolution would take place. When the first bullet was fired, everything got out of control and blood had to be shed.
Revolution was not a necessity for democracy, England's smooth and moderate development is a good example.