The only advantage that I know of is that bandwidth is doubled. Then again, I'm not in the market for a new video card so I haven't really bothered researching the improvements.
A PCI-e 3.0 card will be backwards compatible with PCI-e 2.0 slot. The PCI-e 2.0 slot still has enough bandwidth to any Radeon HD 7xxx series and Geforce GTX 6xx series card and most likely their immediate successors.
Similarly, a PCI-e 2.0 card will work in a PCI-e 3.0 slot.