The cables for USB 3.0 are more expensive and there are effectively an extra two pairs of wires inside each one and new connectors. Cable length is also expected to be limited for maximum speeds and power but this is unlikely to trouble most people. Apart from the increased expense there isn’t much to detract from the new standard.
USB has definitely become the standard connection method for all sorts of devices. With well over 6 billion USB 2.0 devices already sold it is likely that USB 3.0 will prove to be even more popular. Early in 2010 we will see a much greater range of products which support the new standard.