I'll add my voice to the chorus of folks gushing about how awesome this is, but here's a list of reason on just WHY this is so awesome.
Straight off the top, this is only the third launch of the Falcon 9. This rocket alone represents a dramatic disruption of the entire orbital launch industry, offering to make launch of both humans and cargo cheaper than it's ever been by an incredibly wide margin. If this rocket proves to be a reliable workhorse and proves that the existing cost structure is workable then it will both dramatically lower the cost of access to space and drive billions of dollars of revenue to SpaceX.
Second, this represents a significant advance in being able to deliver supplies to ISS, largely replacing a vehicle that is vastly more expensive than the Dragon Cargo. And finally putting American spaceflight back in the game of ISS resupply post-Shuttle.
Third, the Dragon Cargo shares an enormous amount of heritage with the manned Dragon capsule in development. As they prove out elements of the vehicle design in unmanned ISS resupply missions they are effectively proving out the manned version as well. The manned Dragon capsule will represent a huge leap for the US space program, catapulting us back into having the capability to send crew into orbit and to the ISS.
Fourth, given the costs of Falcon/Dragon flights they should be enormously disruptive to manned spaceflight, representing a vastly cheaper way to put humans in orbit even than the Russian Soyuz. That will likely translate into far more people beingable to go into orbit and the development of a sustained orbital tourism industry on an impressive scale which will fuel further innovation and drive down cost of access to space even more.
Finally, all of this is being done by a private company working toward its own ends rather than a company on a strict government contract. This represents a new paradigm for launch vehicles and for manned spaceflight, potentially transforming the nature of the entire endeavor into something that is self-sustaining regardless of government enthusiasm for spaceflight.
In short, this represents nothing less than fundamentally rebooting the entire space program going all the way back to Sputnik, and it could quite readily represent the opening up of spaceflight for commercial applications, space science and exploration, and for manned exploration and colonization to a degree that will make the scifi dreamers of our past look like forecasters.